BOSTON — Philadelphia 76ers center Joel Embiid told his teammates, "I'm back," upon accepting the NBA's Most Valuable Player award on Tuesday night, but 24 hours later, the feels flipped from uplifting to concerning over the course of a blowout Game 2 loss to the Celtics in the Eastern Conference semifinals.

Two weeks removed from spraining the lateral collateral ligament in his right knee during the first round of the playoffs, Embiid scored just 15 points on 9 shot attempts and added 3 rebounds, all well less than half his regular season averages, in the 121-87 loss to a Celtics team the Sixers beat without him in Monday's Game 1. Embiid did not register an assist and committed three turnovers in 27 minutes of action.

Prior to the game, Philadelphia coach Doc Rivers said, "I can guarantee you, if Joel is playing, he's healthy," and "we're not going to risk him for anything," but it was difficult to tell whether injury or rust played a greater role in his lackluster night. Rivers leaned to the latter, but his players were less certain.

"Obviously, he's not 100%, but at the end of the day, he's out there and he's giving his all, and that's all we can ask for," said Sixers forward Tobias Harris, who logged team highs of 16 points and 7 rebounds.

"I was supposed to be out for 4-6 weeks or something like that, so I'm not going to be 100% or fully healed for that whole time," added Embiid, whose self-assessment means he will not be at his best in this series. When he says, "I'm going to keep playing through anything. We've got a big chance, and I've got to take advantage of it," that admirably sounds like a player making a title shot the priority in game-time decisions.

Boston tested Embiid's fitness from the jump. Jaylen Brown curled around a screen and drilled a 3-pointer over a sagging Embiid on Philadelphia's first defensive possession. On the Celtics' next trip down the floor, Marcus Smart drove, drawing Embiid, and then fired a pass to Al Horford in the left corner, forcing Embiid to defend 24 feet. He could not. Horford missed his 3, but Boston would make 20 of them on 51 attempts.

"I've got to be more active, but over the next couple days I'm going to get back to myself as much as I can," said Embiid, willing himself to health. "I thought protecting the rim, it was fine, and I thought it was just the 3s and not being able to keep the ball in front of us. If you can't keep the ball in front of you, that's going to create rotations, and now we're giving up wide-open 3s, and that's how they took advantage of it."

Embiid fell to the ground three times in the first half — once to flop, once when Smart pulled the chair on him and once back-pedaling on defense. He only briefly checked on his knee after the third fall. As he did warming up before the game, Embiid moved enough like himself to convince us he was fine. He was not.

"Obviously, even if it hurts, I try my best and just focus on playing hard and not thinking about what's going to happen if I do this move or move this way," he said. "Sometimes you don't get that response from your body, and sometimes you do. When you don't get that response from your body, you've just got to adjust."

Horford and Grant Williams made sure Embiid felt their combined 476 pounds, battling for position every time the Sixers center sought the ball in the high post. The Celtics did not feel the same force from Embiid on the offensive end through the game's first 21 minutes, as his first three field-goal attempts came from 17 feet and beyond. When Embiid managed to get position in the post, Boston sent him to the free-throw line.

"I'm not making excuses for him," said Harris, "but the playoffs, second round, intensity, physicality, all those things keep upping, and he was really just trying to feel himself out out there and see what felt right."

Embiid's first-half defense was a different story. He registered five blocks and deterred several more drives once his rejection of a Brown dunk attempt proved his knee would not prevent him from protecting the rim.

It was not until late in the second quarter that Embiid made his customary presence known on offense, finishing a pocket pass from James Harden in traffic. Then, inside of a minute before halftime, Embiid took Horford off the dribble on the left side and threw down a monstrous dunk to draw the 76ers within 50-44.

That was the extent of his impact. The game caved in on the Sixers in the third, when Boston's Malcolm Brogdon made 3 of his 6 3-pointers during an all-hands-on-deck, long-distance barrage. Rivers called timeout to stop the bleeding with 2:19 left in the quarter, during which an official review removed an Embiid bucket that failed to beat the shot clock a minute earlier. The air came out of the newly crowned MVP's return. His night was done. Boston owned an 83-60 advantage and built the entire lead on Embiid's watch.

"Early on, they realized Joel was there, and after the third or fourth blocked shot they kept driving," said Rivers. "The difference is they were kicking it out, and we didn't do a good job of scrambling back out."

As the Celtics pushed their lead to 36, Philadelphia's focus turned to Thursday, when doctors will examine how Embiid's body responds to the stress on a knee that everyone but Rivers accepts is not fully healed.

"Swelling is the biggest thing," said Embiid. "You've just got to cross your fingers and hope that it doesn't."

In the meantime, the Sixers will try to strike a balance between what worked in Game 1 — spacing for Harden, whose 45 points on 30 points in the series opener cratered to 12 on 14 in deference to Embiid — and reincorporating the reigning MVP. If their demeanor in the aftermath of the loss were any indication, they are hopeful some combination of shaking off Embiid's rust, matching Boston's effort, manufacturing more than 13 assists and closing a 42-point gap at the arc can hold the home-court advantage they stole.

"We'll fix it," said Embiid.

"It'll be easy to see on film," added Rivers, "and it'll be easy to fix."

Game plans might be. Injuries? Not so easy to fix.

"If I would've come back in Game 3, I'm probably rusty and not myself, but I feel like I just got this out of the way," said Embiid. "I'm disappointed by the loss, but that's a step towards getting back to myself. Obviously, I've got a lot of work to do. That starts tonight and tomorrow to make sure I'm ready for Friday."

Fingers crossed.