Joel Embiid admitted his shooting was way off where it should be as the Philadelphia 76ers star warned his COVID-19 road to recovery could be a long one.

Coach Doc Rivers is braced for the possibility there could be more tough nights to come for Embiid after he struggled in an 88-87 defeat to the Boston Celtics.

MVP runner-up Embiid had a double-double of 13 points and 18 rebounds, but he was just three-of-17 shooting.

His points tally was his lowest in a game this season, and Embiid's average of 22.0 points is way down on last term, when he averaged 28.5 over 51 games.

He has plenty of time to get that figure up, but Embiid is working his way back from a nine-game coronavirus lay-off, and despite scoring 42 points against the Minnesota Timberwolves in his comeback game, he has followed 16 against the Orlando Magic with another disappointing performance.

"I would never use it as an excuse, but obviously I think it's going to take me a while to get back, especially legs and cardio and all that stuff," said Embiid.

"But every single day I've got to keep working hard, and keep getting better. I can't be playing the way I've played in these last two games, especially when it comes to shot-making.

"If teams are going to double me and I'm not going to get easy baskets, I have to create for myself and my team-mates, and I've got to make those shots."

The Sixers (11-11) head to the Atlanta Hawks on Friday and then travel to tackle the Charlotte Hornets in two games early next week.

Rivers can see Embiid is not quite himself on the court, but the coach sees no other way through the situation that continuing to play and support his Cameroonian star.

"We've got to get Joel going – number one," Rivers said. "We get him going, the whole team gets going. That's the number one thing.

"But that's two games in a row I thought the offense was lacklustre. Execution is really bad right now. But, again, I'm not concerned by it. We're going to get through it. It's just that we're dropping games while we're getting through it."

The Sixers shot just 8-of-31 from three-point range and were 33-for-89 overall, Seth Curry leading scorers with 17 points.

Rivers appreciates Embiid is lacking rhythm, but the coach cannot put a finger on quite why it is the case.

"Everybody with COVID, I just don't know those answers," Rivers said. "Some guys come back quickly and play pretty well, a lot of guys have played the one great game when they're back and then they've gone into a little fog.

"You could tell he's definitely struggling getting going. We've just got to keep playing him and let him play through it."