BOSTON – With Jaylen Brown unable to play, some thought the already undermanned Boston Celtics would be easy pickings for the 76ers.

That was far from the truth, as the Celtics were anything but helpless.

If fact, the Sixers could have used some bodies, as Boston took a 117-101 victory in Game 1 of the Eastern Conference semifinals Monday night at TD Garden.

"I think they came ready to play, and we were very rusty," said Ben Simmons, whose squad had been idle since last Tuesday. "We just got smacked multiple times and we didn't react. I think that's what it was."

Brown missed the game with a strained right hamstring. The Celtics were also without Kyrie Irving (left knee), Gordon Hayward (left ankle), and Daniel Theis (left knee), who all suffered season-ending injuries.

But Jayson Tatum, Al Horford, and Terry Rozier more than made up for their absence.

Rozier had 29 points while making 7 of 9 three-pointers. Tatum had 28 points, and Horford added 26. The all-star post player made 10 of 12 shots.

Tatum took advantage of being primarily guarded by JJ Redick rather than Simmons or Robert Covington.

"We'll look at that," coach Brett Brown said. "We'll go back and look at that. For different reasons, we decided to do that, and well go back and see how that worked."

As good as the Celtics were, the Sixers' repeated defensive miscues also contributed to the outcome. A prime example came with the Celtics up, 97-88. Shane Larkin and Horford orchestrated a pick-and-roll. Redick, who was guarding Larkin, went under Horford's pick before picking up the reserve point guard again. Joel Embiid also followed Larkin, leaving  Horford wide-open.

Larkin found the big man, who buried a three-pointer to put the Celtics up by 12 points with 5 minutes, 32 seconds left.

"We missed a lot of coverages, didn't execute offensively like we wanted to every time down the floor," Simmons said. "But that's on us."

The Celtics hit 17 three-pointers, shot 48.2 percent (41 of 85) from the field, and made 18 of 19 foul shots.

"It starts with defense," Embiid said. "I thought I was [lousy]. I thought we were all bad tonight.  That's not who we are defensively."

Boston was more aggressive. Asked if the Celtics frustrated them, Simmons said nothing like the Miami Heat. The Sixers' opening-round series with the Heat was physical. Players had to be separated on occasion.

On Monday, the Celtics were physical with Simmons. On one inbounds play, Marcus Morris shoved the rookie point guard in the open court.

Embiid finished with a game highs of 31 points and 13 rebounds. Redick had 20 points but made just 2 of 7 three-pointers. Simmons had 18 points, seven rebounds, six assists,  and seven turnovers. Covington struggled, scoring three points on 0-for-6 shooting, missing all four of his three-point attempts. The Sixers shot 19.2 percent (5 of 26) as a team on three-pointers.

The Celtics built a commanding 19-point lead late in the fourth quarter.

"To look at this game, defensively, offensively, this isn't who we are," Brown said. "This was a very poor game for us. I give the Celtics a lot of credit producing that. This isn't who we are."

Jaylen Brown was sidelined after straining his right hamstring in the first half Saturday against the Milwaukee Bucks. He didn't play in the second half of that Game 7. He was listed as doubtful heading into the game Monday night.

To combat Embiid, the Celtics inserted center Aaron Baynes back into the starting lineup. Semi Ojeleye had started at power forward in the previous three games, with Horford, the normal power forward, moving to center. However, Horford started at power forward Monday night, while Ojeleye came off the bench.

Baynes did a solid job of defending  Embiid in the past.  It's hard to say if the all-star had a tough time with the 6-foot-10, 265-pounder  or Embiid was just rusty.

Embiid missed two of his first three baskets before heading to the bench with 7:11 left in the first quarter.

He wasn't the only Sixer to struggle.  They missed 14 of their first 18 shots and went 1 for 6 on three-pointers en route to trailing, 21-13, with 3:14 left in the quarter. In addition to missing shots, the Sixers had a tough time stopping Rozier early.  He had 10 of the Celtics' 21 points on 4-for-7 shooting, including both of his three-pointers.

The Sixers, however, finally woke up and went on a 9-2 run to close the gap to one point,  23-22.  The Celtics ended the quarter up 25-22 after North Philly native Marcus Morris' alley-oop dunk at the 59.6-second mark.

Philly took the lead,  29-27, on Embiid's jumper  with 9:32 left in the quarter. The lead changed three times before the Celtics built a 56-43 lead on Rozier three-pointer 27.5 second before intermission.

Boston had exploited mismatches all half, and  Tatum (16 points), Rozier (13), and Horford (12) all scored in double figures in the first half.  Embiid (13) and Redick (10) were double-figure scorers at the half. However, the Sixers shot just 3 for 15 on three-pointers in the first half.