Before the NBA playoffs, the consensus was that the Eastern Conference had three true contenders in the Boston Celtics, Philadelphia 76ers and Milwaukee Bucks. And some people gave the Cleveland Cavaliers a fighting chance as well.

Just one round in, the picture looks much different. The Bucks are done after a stunning 4-1 series defeat to the Miami Heat. The Cavaliers are gone too, suffering a 4-1 defeat to the New York Knicks. And, while the Heat and Knicks proved they are both formidable foes that can’t be taken lightly, the winner of the Celtics-76ers series will likely be the favorite to come out of the East (more on this later).

The series was already an exciting matchup between the likely MVP and the reigning conference champions. But with the stakes elevated after Milwaukee’s defeat, there is an even more heightened sense of anticipation for the series between the two longtime rivals.

To preview this heavyweight battle, we’ve provided the five most important storylines heading into Game 1 of the series at TD Garden.

(Sidebar: Stick around until the very end to see what our playoff projection model thinks of this series!)

1. When Is Embiid Coming Back?

This is both the most important question of the bunch by far and the least interesting to break down. Joel Embiid is doubtful for Game 1 and may miss multiple games after that with an LCL sprain.

If Embiid misses two games, it’s a serious blow for the 76ers, but not quite a death sentence. Philadelphia will undoubtedly put the ball in the hands of Tyrese Maxey and James Harden a ton and hope for some positive outlier shooting games. Defensively, the Sixers will have to defend better than usual at the point of attack without Embiid erasing mistakes behind the play as he so often does.

The Celtics will be heavy favorites in any game Embiid misses, but in a two-game span, anything can happen. The 76ers could get hot and steal a game, and – worst-case scenario – if Embiid is back for Game 3, they will still have the chance to hold serve at home.

The longer Embiid sits (or plays while seriously compromised), the harder it is to envision a path for a Philadelphia upset. Boston is already favored, has more playoff experience, and depth that Philadelphia can’t match.

With Embiid out, Boston can basically utilize any lineup it wants without fear of having to have someone in to at least slow down the big man. For entertainment’s sake, let’s hope Embiid can make a quick recovery because this has the makings of a fun series with him on the court.

2. How Can the Celtics Slow Embiid?

If Embiid’s health doesn’t slow him down, the Celtics are in the unenviable position of trying to stop the runaway freight train. The star center has always been lethal, but he’s taken his game to a new level this year.  

He’s still a monster in the post, but he’s starting more sets at the nail this year. That way, he’s able to see the floor better, use his long strides to get around any defender playing him too closely, or fire off a midrange jumper that has become automatic unless contested. It also makes it more difficult to send a second defender at him.

Like all great players, Embiid will destroy any coverage that he sees too often, so the Celtics will have to vary coverages and send help from different places to try and confuse him. In his last meeting with Boston, Embiid had the best game of his career with 52 points on just 25 field goal attempts and 13 free throw attempts. He had six assists and three turnovers.

He won’t be able to put up those numbers every game, but the 76ers will need him to have a monster series to win. He coasted at times against the Brooklyn Nets in the opening round and was content to give the ball up when even soft double teams came because the team was good enough to win with him doing that. That won’t be the case against the Celtics.

Superstar players are going to get shots, and, if they aren’t off their game, will score points. Embiid is no different. But the Celtics must decide which shots they can live with. The 76ers have a good team around Embiid, but he’s a different player than, say, Nikola Jokić. You could make the argument you’d rather have Jokić go 1-on-1 and limit his passing. With Embiid, it’s about limiting his quality looks.

This doesn’t just mean shot location, but the ease of the shots as well. Yes, they need to limit Embiid’s shots at the rim and do their best not to send him to the line, but they need to make every shot difficult. If he’s comfortable in the midrange, it’s curtains. Notice the difference in the ease of the shots below, even though they’re both makes.

Blake Griffin can’t handle Embiid’s burst to the rim, so he plays off him. In doing so, he lets Embiid walk into an open midrange with his feet set. He’s going to make that way too often.

Meanwhile, the second shot goes in, but Horford gets a contest in, and Derrick White is there to bother Embiid if he wants to continue going left as well. That defense is enough to force a few more misses, which could be the difference between victory and defeat.

The good news for Boston is that there will be much more Al Horford and Robert Williams guarding Embiid and much less Blake Griffin and Luke Kornet than there was during the 52-point game. But Embiid can walk into easy jumpers against Boston’s starters too, and they can’t let him settle into a comfort zone there.

The 76ers are not a great offensive rebounding team, grabbing the fourth-least offensive rebounds per game in the regular season. So the Celtics should be aggressive showing late help against Embiid. But there’s a difference between late help and too-late help.

If Jayson Tatum is the help here, he has to come earlier. Embiid is the last guy in the league you want to help late against. He’s too big and strong to do anything with when he’s already coming downhill. Just come early, force him to make a pass to Tobias Harris in the dunker’s spot and live with the result.

The Celtics need to be aggressive in putting multiple bodies between Embiid and the rim early, forcing him to make tough passes and his teammates to make looks going to the rim.

3. Can Philly Contain Boston’s Shooting?

It’s a make-or-miss league, so it’s easy to say that the 3-point shot is a difference in every series now. With how many 3s are taken in every game in the modern NBA, a few more makes can easily be the difference between winning and losing for any team.

But Boston isn’t any team. It’s one of the best at generating open looks and knocking them down. The Celtics took the second-most 3-pointers per game this season and had the sixth-highest 3-point percentage. The team blended volume with efficiency, and it’s the biggest reason its offense is elite.

The challenging part in terms of defending the Celtics is that their entire rotation except for Robert Williams shoots 3s. Nine Celtics are playing 10-plus minutes per game in the postseason, and all of them averaged 3.7 or more 3s per game during the regular season. Five of those eight shot 38% or better from beyond the arc. If there’s an open jumper to be had, the Celtics will take it and probably make it.

Philadelphia did a good job against shooters in the regular season (fifth in the league in opponents’ 3-point percentage), but Boston generated good looks in the regular-season series. The Celtics shot 41.1% combined in the four meetings. If that holds, the 76ers’ chances of winning the series are slim.

The biggest question for the 76ers is how often the team can stay out of rotation. Are the Sixers content with letting Tatum and Jaylen Brown get shots off with less help, or will they send help every time one of them gets an advantage? How early will the help come?

The Celtics destroyed the Atlanta Hawks on isolation plays in the first round. Boston averaged 1.1 points per possession on isolations in the first round, per – the second most among teams in the opening round.

But the Hawks had Trae Young on the floor and didn’t have Embiid helping on drives. Boston was only 16th in the regular season in points per possession on isolation. Unless Boston gets a huge advantage in a matchup, Philadelphia should send some help, but not to the degree Boston should against Embiid.

The matchup Boston will try to exploit is Tyrese Maxey against either Brown or Tatum. Tobias Harris and James Harden are both physical enough to at least bother these players if the switch happens late enough on the shot clock. (Harden, for all his defensive shortcomings, is still stout in the post, and will at least be attentive in on-ball situations).

Look for Boston to use whoever Maxey is guarding as the screener for Tatum late in games. Philadelphia doesn’t want to switch this, and it hasn’t been sharp enough at the point of attack to bother Tatum in these situations.

Yes, Grant Williams misses the initial shot here, but a simple screen and dive from Marcus Smart brings four different 76ers to the ball and leaves Smart with two separate players wide open in the corner for 3-point attempts. Corner 3s are the best shots in the league besides layups and Boston generates a wide-open attempt here without much effort.

Even if they can’t take away all the open 3s Boston gets, the Sixers must make a concerted effort to cut off the corner. Boston was good everywhere, but especially deadly in the corners, shooting 40.4% on the year. Philadelphia finished in the top 10 in the least attempts allowed from the corners and the lowest 3-point percentage allowed from the corners during the season.

They need to keep up that corner 3 prevention to keep Boston’s offense from overwhelming them.

4. Can the Sixers Leverage their Advantages?

Even if the Celtics defend Embiid correctly, it’s going to leave leverage for the other players on the team. Can the two most talented creators other than Embiid take advantage?

Harden was brought in to be the second star, so it seems natural to assume he has to be the one to rise to the occasion. But even discounting Harden’s checkered playoff past, he shot 26.5% on 2-point attempts in the first round, and the burst he’s known for in getting to the rim hasn’t been there.

He can still knock down 3-pointers and set up teammates, but if Boston forces him into contested looks at the rim and can limit the number of fouls, Harden is going to have a tough time getting going. Though he’s still going to be an important factor, he may not be the one who can use the leverage Embiid gains effectively.

Maxey has been that guy for some time now and was actually Philadelphia’s leading scorer in the first round. However, he really struggled in the last three games versus Boston during the regular season, averaging 6.3 points on 28.1% shooting.

The greatest thing about Maxey’s ascent has been his decisiveness. Defenses have focused on Embiid and Maxey is a lethal shooter. These two things have combined to give him driving lanes that should not be given to someone with his lightning quickness.

All Embiid does here is set a screen that Maxey doesn’t use. But Smart is so concerned about getting over it that he overplays Maxey before Maxey even starts to go that way, and Noah Vonleh is so worried about helping off Embiid that he never steps in to properly contain Maxey’s drive. It’s an easy layup.

These plays were few and far between in the last three matchups against the Celtics, but they’re the kind of openings Maxey needs to look for if Philadelphia is going to be successful on offense. He’s going to have to generate some easy looks against this tough defense. If he can, Harden can focus more on being a spot-up player and a playmaker off 0-2 dribbles. Harris can focus on being a tip-of-the-spear weapon. Everyone else’s roles fall in line.

If Maxey struggles, the burden on Embiid will get even bigger, and Harden will be forced to try to bend the stout Celtics defense. It likely won’t end well.

5. How Big of an Advantage Is Boston’s Bench?

The Celtics rotation is already firmly established one round into the playoffs.

They’ll likely use the same starting lineup with Robert Williams and Malcolm Brogdon coming off the bench. Grant Williams and Sam Hauser will essentially combine for the last rotation spot, with Joe Mazzulla opting for whoever is playing better.

Robert Williams is going to be huge in a series against Embiid, providing an extra body to cover him and vertical spacing at the rim. Brogdon won the NBA Sixth Man of the Year for the first time due to his ability to fit in any Boston lineup and provide playmaking, spacing and defense. Grant Williams can be erratic but has had huge playoff moments and Hauser is there as the emergency shooter to jumpstart the offense.

De’Anthony Melton is a solid contributor and an important X-factor in this series. He’s an excellent defensive guard who can shoot and be trusted in a low-usage role. But that’s where the certainty of the 76ers bench ends.

Paul Reed has had some moments and filled in remarkably in Game 4 against the Nets when Embiid sat. Bball Paul had eight offensive rebounds and played good defense to help Philadelphia close out the sweep. But, while he’ll be an important factor when Embiid is out, he’ll likely play 10 minutes or so when Embiid is playing. So he can be a solid and important contributor, but his impact is limited.

Jalen McDaniels was acquired for this specific series, giving the Sixers another wing defender to throw at Tatum and Brown. However, he’s been mostly invisible on offense since joining the team, and it’ll be hard to play him alongside P.J. Tucker at any point unless Embiid is planning on having a usage rate of 100%.

But the other options aren’t promising, either. Georges Niang is a lethal offensive player, but he’s the worst defender in Philadelphia’s rotation by far by both DRIP and the eye test. Even if Doc Rivers doesn’t trust McDaniels offensively, it’s going to be hard for Niang to play extended minutes in a series without a real place to hide him.

Danuel House hasn’t helped the 76ers as much as they hoped this year. Maybe if Rivers gets desperate he dusts off Shake Milton, who has been in and out of the rotation this year and is currently in the definitively out category. The most likely scenario will be that Rivers pushes his starters as much as he can and prays McDaniels makes enough shots to stay on the court.

Boston has more answers off the bench than Philadelphia does. How wide the gap in bench play is could go a long way to deciding the series.

Who Has the Edge?

In terms of the moneyline, sportsbooks have the 76ers as heavy underdogs (+350) and the home team Celtics as the clear betting favorites (-500). But what is our model’s prediction for who will advance to the conference finals?

This model calculates each team’s chances of making it to the semifinals, conference finals, NBA Finals and winning the title outright, based on 5,000 simulations of the playoffs.

It incorporates our adjusted team ratings (including overall adjusted team rating, adjusted offensive rating and adjusted defensive rating), accounts for recency bias (so, it gives more weight to teams that enter the second round playing well rather than those who stumbled through the first round) and for how well teams performed against other good teams (because in the playoffs you typically have to beat them).

Here are the model’s projections in real time. These will updated throughout the series:

Probabilities of Advancing to Each Round of the NBA Playoffs

Check out our Denver Nuggets-Phoenix Suns, Miami Heat-New York Knicks and Los Angeles Lakers-Golden State Warriors predictions. And don’t forget to follow us on Twitter.