1:15 AM ET

BOSTON -- The Boston Celtics missed an opportunity to take control of their Eastern Conference semifinals matchup against the Philadelphia 76ers, dropping Game 1 in Boston on Monday night 119-115 despite MVP candidate Joel Embiid being sidelined with a sprained right knee.

The Celtics shot 58.7% from the field and got 39 points from Jayson Tatum, but Boston couldn't match the same intensity on the defensive end, allowing Philadelphia to shoot 50.6% from the field. The Sixers were also carried by James Harden, who put up 45 points and hit the go-ahead 3-pointer with 8.1 seconds remaining.

"I felt like we started off well offensively, but I don't think there was a sense of urgency on the defensive end," Celtics center Al Horford said after the game.

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"Embiid being out tonight, as much as you want to say it wasn't a factor, I think it was. We have to be better. We have to understand that we have to be able to play with whatever happens, whatever variables. With our team I've just seen it happen with us for whatever reason."

While Tatum said he did not think the Celtics underestimated the Sixers without Embiid, he did agree that Boston's intensity on defense was not at the level it needed to be.

"I think we allow guys to be a little bit too comfortable at times," Tatum said. "We definitely can be better. Sometimes we give too many offensive rebounds and that hurts us, second- and third-chance opportunities. It's all about picking up our pressure and trying to make people uncomfortable ... everyone is talented and they feel a little bit more free out there."

This is the third time the Celtics and Sixers have faced each other in the playoffs in the past six seasons, and Boston entered this series 8-1 against Philadelphia in those matchups. However, Celtics coach Joe Mazzulla noted how this year's Sixers had managed to form an identity without Embiid on the court, playing faster and spacing the floor with Harden running the offense. Philadelphia was 12-5 in games Embiid missed this season.

Still, Boston entered Game 1 as a double-digit favorite and took advantage of Embiid's absence by attacking the basket and scoring in the paint at will (Boston outscored Philly 66-42 inside).

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But in the fourth quarter, the difference was Harden, who outscored the duo of Tatum and Jaylen Brown 15-10 during the final period. Tatum's final made basket came with 11:06 remaining in the fourth quarter. Brown's was at 6:11.

"Give the other guys credit," Tatum said while looking at the stat sheet during his postgame news conference. "Harden had 45. Melton hit 5 3s. Those other guys stepped up and played big and knocked down shots.

"Regardless of the circumstance, you want to win, and we didn't do enough to win. Every game you lose is a missed opportunity to get a win."

The Celtics lost the lead down the stretch following a turnover from Malcolm Brogdon on a pass thrown directly to Sixers guard Tyrese Maxey with the shot clock winding down. Tatum said the Celtics thought a shot clock violation had been called on the play and they did not chase Maxey down the court in transition, as he scored a basket with 28.2 seconds remaining in the fourth to put the Sixers in the lead.

Even though Tatum knocked down a pair of free throws on the other end, it only set the stage for Harden and the Sixers to steal Game 1 in Boston and stun the Celtics, who let an opportunity slip away.

"Not concerned but I do have a level of awareness," Brogdon said. "We got to be ready. We got to come out ready. We got to come out and send a message next game."