"You've got to have role players to have a good team," Celtics coach Doc Rivers said.
House and Posey played their roles to near perfection. The duo went 7-for-13 on 3s that made all the difference late in the game against a Sixers team with something to prove. Posey scored 17 points, House had 15 and Boston's bench outscored Philadelphia 46-26.
"We were just making plays for each other and that helped us in the second half to come back, get a lead and get a win," Posey said.
Pierce had 19 points and 12 assists, Garnett scored 22 points and Allen scored 12.
Andre Miller kept the Sixers alive until the fourth with his first double-double of the season, a 26-point, 12-assist effort that was one of his best games in Philadelphia. Andre Iguodala scored 24 points.
The Sixers gave an inspired effort in their first game with Ed Stefanski as president and general manager. Stefanski left his job as general manager of the Nets to take a similar role in Philadelphia after Billy King was fired on Monday.
"For 40 minutes we were there, but you've got to be able to sustain it against a team like that," coach Maurice Cheeks said.
Posey hit a pair of 3s in the final minute of the third that helped the Celtics take a three-point lead. He hit another early in the fourth that tied the game at 86.
The Sixers went ahead one last time, 88-86, on two of Miller's free throws. They played hard, played smart, even shot 56 percent from the floor, but they simply couldn't stop the Celtics down the stretch.
House hit consecutive 3s that sparked a 12-2 run that made it 98-90. Posey hit another 3 later in the fourth that pushed the lead to 103-94 and the Sixers did not have another run left in them.
"Those are big. They're back breakers," House said.
Boston and Philadelphia, once the fiercest of rivals, exchanged baskets in the third quarter to make this one a fun, competitive game.
"We can play with them," said Samuel Dalembert, who scored 13 points. "We just need to learn how to keep a lead. We can't just let them get their confidence going."
More performances like the Sixers had in the first half and King might still have a job. The Sixers entered shooting 43 percent from the floor, a number that hardly seemed to suggest they'd make 14 of 19 in the first quarter and take a 32-26 lead.
"I don't think they sense how excited people are to play them," Rivers said. "We've never gone through this. Teams are up to play us."
Iguodala's perfect lob to Dalembert set up an alley-oop in the second, and Iguodala hit a fallaway jumper as the shot clock expired to get a rise out of a rare strong home crowd. That shot gave the Sixers an 11-point lead and they led 57-52 at halftime.
"Maurice has those guys playing hard every night," Rivers said. "They don't act like their record. To me, that's all Maurice. They still play hard for him. It wasn't nice for us, though."
Pierce scolded a couple of teammates in the locker room who having too much fun and screaming to some music before the game. "We're trying to play a game," he said. The two players got quiet. ... Pierce said the additions of Garnett and Allen mean he no longer has to play the role of big brother. He said last year he spent too much time telling his younger teammates what to do, where to go, what to eat, how to dress. ... King met with the media earlier in the day and said he was surprised at the timing of his firing. "I don't regret rebuilding," King said. "I think it was the right thing to do."