BOSTON -- The Boston Celtics picked an inopportune time to have their worst overall game of this postseason.

A 115-103 loss to the 76ers in Game 5 on Tuesday night put Boston in a 3-2 series hole with Game 6 in Philadelphia on Thursday night.

"We dropped the ball tonight," Celtics guard Jaylen Brown said. "Couldn't stay in front of anybody. Couldn't get a stop when we needed to. Missed a lot of wide-open shots, gave everything up that they wanted us to give up. That was the story of the game. But got to have a short-term memory."

The Celtics never led after midway through the first quarter, shot just 39.8% from the field and finished with their lowest point total (103) of these playoffs. They shot 6 for 22 against the Sixers' zone defense and 4 for 15 overall when contested by Joel Embiid.

Meanwhile, the Sixers shot 50.6% from the field, as Embiid finished with 33 points, and Tyrese Maxey had 30. James Harden added 17 points and 10 assists, and Philadelphia shot 10-for-17 off his passes.

"We just didn't have it today," said Celtics star Jayson Tatum, who finished with 36 points after starting 0 for 5 in the first quarter.

Added coach Joe Mazzulla: "That was our first really, really bad game of the playoffs. It doesn't come at necessarily the best time, but we just have to shift our perspective and we've got to get ready to play the next game."

The Celtics left their previous two defeats in this series -- a 4-point loss against the Embiid-less Sixers in Game 1 and an overtime loss in Game 4 -- lamenting missed opportunities. They had chances to win both and entered Tuesday vowing to be better with their late-game execution.

But after losing two games it had an opportunity to win, Boston was bombarded by Philadelphia from the start in Game 5.

"There's nothing we can do about tonight," Tatum said. "It happened. It's unfortunate. We wish we could change it, but that's not how life works. Can't go back in time."

The Celtics scored 72 points and trailed by 16 entering the fourth quarter -- their lowest point total through three quarters and their largest fourth-quarter deficit at home through the regular season or postseason. Their uninspiring performance led the sold-out crowd at TD Garden to boo loudly on several occasions throughout the night.

"The past is the past. We can talk about a lot of stuff that's happened in this series that could have went the other way, and it didn't," Brown said. "Right now, we've got in front of us a great opportunity. To mull in the past would take that opportunity away from us."

The Celtics now face elimination, but have fared well in such scenarios in the past.

They've won six best-of-7 series when trailing 3-2, more than any other NBA franchise. That includes the conference semifinals last season against the Milwaukee Bucks, when they lost Game 5 at home but responded with a road win and then a home rout.

However, Brown didn't want to take solace in that comeback Tuesday night.

"Last year is over with," he said. "This year we got to come out and be better than we were tonight, or we'll have a different ending. Obviously, we still are in this series, and we got to muster up whatever we got left to be better for Game 6."

ESPN Stats & Information contributed to this report.