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Ever since the Boston Celtics and Philadelphia 76ers opened the 2022-23 NBA season, the two rivals seemed destined to meet in the playoffs.

Here we are.

After the Celtics dispatched the pesky Atlanta Hawks in Game 6 Thursday night -- and 11 days after the 76ers swept the Brooklyn Nets -- they will officially face the 76ers in a true clash of the titans in the Eastern Conference semifinals beginning Monday night inside Boston's TD Garden.

Game 1 will be the fifth meeting between them, with Boston winning the season series 3-1. (The average margin of victory was just 5.25 points.) Philly's lone win, however, featured a 52-point, 13-rebound performance from Joel Embiid.

This will be the 22nd time the Celtics and 76ers will square off in a playoff series, the most between any two teams in NBA postseason history. And as the two teams prepare for one of the most anticipated series of these playoffs, here is what could determine who reaches the conference finals, including the health of the MVP favorite, the Celtics' massive lineup decisions and the importance of the Sixers' guards.

The health of Joel Embiid

Embiid left Game 3 of Philadelphia's sweep of the Nets after an awkward fall trying to contest Nets forward Cam Johnson's shot.

Embiid suffered a sprained LCL in his right knee on the play, an injury that caused him to miss Game 4 against Brooklyn - after which 76ers coach Doc Rivers said Embiid would be "probably 50 percent, at best" to play in Game 1 against Boston.

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The 76ers were actively rooting for Atlanta to extend its series with Boston to six games in order to give Embiid a couple extra days of rest by pushing back the start of this series to Monday. So there's little doubt there were plenty of cheers in Philadelphia when Trae Young led the Hawks to a dramatic come-from-behind win in Game 5.

Now, it will be a race to get the superstar center healthy - one that he's now got a couple of extra days to run before it ends. And, of course, there's still the matter of just how effective Embiid will be, or if this will be another unfortunately timed playoff injury in a career that's seen several of them.

Earlier this month, Boston saw first-hand what a healthy Embiid could do. He went for 52 points, 13 rebounds and 6 assists in an April 4 win, scoring more than half of Philadelphia's points. In that win, Embiid became the second player in NBA history to record 50 points, 10 rebounds, 5 assists and 80% shooting in a game, joining Wilt Chamberlain, who did it twice.

James Harden's 2s

Across the four games against Brooklyn, Harden shot just 9-for-34 on 2-point shots. Remove his 5-for-8 performance in Game 3 before he was ejected, and that drops down to a truly dismal 4-for-26 across Games 1, 2 and 4.

Harden's numbers were even worse when he drove. Harden shot 7-for-30 (23.3%) in the paint against the Nets. Among players with at least 25 attempts, that's the worst paint field goal percentage in a single series since play-by-play was first tracked in 1996-97, according to ESPN Stats & Information research.

Both Rivers and Harden dismissed his poor shooting, saying he'd simply missed shots he normally would hit. But given Brooklyn is a lesser version of the long, athletic, switchable defense Boston employs, Harden's struggles could become a significant concern.

During the regular season, the Celtics switched 73 times when Harden was the ballhandler for an on-ball screen, their most instances against a single player, per Second Spectrum tracking. Harden actually excelled on these plays, averaging 1.17 points per chance. (The Celtics allowed 0.95 points per chance when switching against on-ball screens in the regular season, which ranked in the top 10 league wide.)

Boston's 3s

The simplest way to know how a game went for the Celtics this season has been to look at their 3-point numbers.

When the Celtics hit at least 40% of their 3s, their record was 33-2 across the regular season and playoffs. When they were below that mark, it was 27-24.

Given Boston averaged 10 more 3-point attempts than Philadelphia during the regular season, the Celtics are already likely to have an edge in perimeter scoring in this series. So if Boston can have a few hot-shooting nights, it will make for an uphill battle for Philadelphia.

Will Boston lean big or small?

The Celtics have one of the league's deepest teams, which affords coach Joe Mazzulla different ways of deploying his forces. And entering this series, there is no bigger question than whether Boston will lean into playing bigger or smaller against Philadelphia and its mountain in the middle in Embiid.

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Marcus Smart, Jaylen Brown, Jayson Tatum and Al Horford are locked in as starters against Philadelphia. But will Mazzulla go with another big in Robert Williams III -- as Boston did at its best last year -- or a small in Derrick White -- which is how the Celtics played against Atlanta.

Two bigs on the court could help to contain Embiid, but the defense-first Williams will allow Embiid to rest on defense. Playing smaller with White could force Embiid to move around a lot more, even if it makes it more difficult for the Celtics to guard Embiid. Which path Mazzulla goes, and how Philadelphia adjusts, will be closely monitored.

Can Maxey get going against Boston?

In his third season, Tyrese Maxey has proven to be one of the most explosive young scoring guards in the NBA.

The Celtics, however, have been his Kryptonite.

Maxey is a combined 17-for-48 across the four meetings with Boston this season, including 9-for-32 over the final three contests. The Celtics have long, rangy defenders to throw at Maxey, including Smart, White and Brown, and can wear him down defensively.

The Celtics limited Maxey to nine transition shots -- he made four -- across the teams' four regular season games. Maxey, among players with at least 100 such attempts, ranked in the top-10 in effective field goal percentage in transition this season, per Second Spectrum.

ESPN Stats & Information's Matt Williams contributed to this story.