The Boston Celtics have had the Sixers’ number. Joel Embiid’s supporting cast must produce for that to change.

The Boston Celtics are the worst imaginable matchup for the 76ers.

Optimists can look at past meetings between the two teams and point out that the Sixers lost their first three regular-season meetings by an average of only 6.3 points. They’ll also note Joel Embiid’s 52-point, 13-rebound effort, which carried them to a 103-101 victory over Boston on April 4.

But they won’t mention the Celtics’ roster in that final game, which was without All-Star Jaylen Brown and elite defender Robert Williams III, or that Boston went through the motions for three quarters.

» READ MORE: Sixers set to face big challenge against Boston Celtics in Eastern Conference semifinals

That won’t be the case when Game 1 of the Eastern Conference semifinals begins at 7:30 p.m. Monday at TD Garden. The Sixers also won’t have the luxury of waiting for Embiid to get healthy any longer. The presumptive league MVP may have to wear a brace on his right knee after spraining his lateral collateral ligament. And that doesn’t bode well for a team that heavily relied on their big man against Boston this season.

While they won’t publicly say it, the Sixers would have preferred to face the Atlanta Hawks in the conference semifinal.

This is the same Hawks team they defeated 136-131 in overtime on April 7 despite resting their entire starting lineup and sixth man De’Anthony Melton. In addition to being successful against Atlanta, the third-seeded Sixers would have had home-court advantage against the Hawks.

Now, a potential winner-take-all Game 7 matchup would be hosted by the second-seeded Celtics since Boston defeated the seventh-seed Hawks in the first round.

» READ MORE: Everything has broken right for the Sixers and Joel Embiid. Now they need to capitalize.

And there are concerns and questions about who will step up and help Embiid.

The Sixers are at their best when they’re moving the ball, getting everyone involved. The problem is, it rarely happens against elite competition, especially Boston. In those games, the Sixers have little variety in their playing style. It’s Embiid or bust.

Tobias Harris tends to be hot early. However, the Sixers often go away from him as the game progresses.

James Harden averaged 25.5 points and 9.9 assists against the Celtics this season. He also gave up a lot of easy buckets on defense. The Celtics knew they could manufacture buckets, and they did so while attacking him. And Tyrese Maxey, for the most part, has been a poor-shooting bystander.

As a result, the Sixers had to get tough buckets. Embiid hit big shots over multiple defenders, which is a credit to the six-time All-Star. But he needs help.

The Sixers coaching staff must find a way to keep Harris involved through four quarters. It also needs to run the offense through Maxey more, especially when they stagger the lineups. The Sixers will need his energy and scoring, so coaches must also put him in better positions. They’ll also need the bench to step up.

The Sixers are the deepest they’ve been in Doc Rivers’ three seasons as coach. However, their bench doesn’t compare to the Celtics’ second unit, which is led by sixth man of the year Malcolm Brogdon.

That became obvious in the Sixers’ 110-107 loss to the Celtics on Feb. 25. Boston led, 27-12, in bench points. But even that didn’t show how wide the scoring disparity was that night. Maxey came off the bench that game and scored six of his points in the fourth quarter. Before that, Boston held a 22-6 advantage.

» READ MORE: Tyrese Maxey has struggled against Boston. He expects to play better when they meet in the second round.

Jalen McDaniels was minus-23 in 14 minutes, 50 seconds. Georges Niang was minus-22 in 12:41 while constantly being attacked on defense. Paul Reed was minus-14 in 4:24. And Shake Milton was minus-4 in 1:48. In this game, Niang and Reed looked unplayable. The Sixers will need both to make contributions to beat the Celtics.

“Clearly, we need to be better,” Niang said following that game. “That’s not acceptable. That’s on us as a second unit. We need to be better and hold down the fort. Whether it’s make shots, take better shots, play better defense. That’s on us.”

We’ll see if Niang, McDaniels and Reed, who are key reserves, can fare better against Boston in a postseason setting. We’ll see how the Sixers, known for struggling against elite perimeter players, defends Boston’s high-scoring All-Star wing tandem of Jayson Tatum and Brown. We’ll see how the sprained knee hampers Embiid.

And most importantly, we’ll see if the Sixers can overcome having their worst imaginable matchup.