BOSTON - It could be the injuries. It could be old age. It could be the 76ers' in-your-face defense.

It could have been any number of things. But the same Celtics who could do no wrong in the fourth quarter of Game 1 of this series couldn't buy a midrange bucket for two full quarters of Monday night's 82-81 loss to the Sixers- not from captain Paul Pierce, not from the previously unstoppable Kevin Garnett, not even from sweet-shooting Ray Allen.

The Celtics scored 25 points in the first 12 minutes of the game. They failed to score 25 points in the next two quarters combined.

"I thought tonight when they trapped us, we tried to reject it and force shots," Celtics coach Doc Rivers said. "I wasn't proud of the way we played offensively. I don't like the fact that, to me, we took almost three quarters to play the right way offensively.

"We knew the blueprint before the game, but it took us three quarters to get the offense going. I always say that's on me."

Rivers needed energy out of his aging team and wasn't finding it from a host of players who were firing brick after brick. Then, within a 60-second span, Mickael Pietrus changed everything and gave Boston hope for another late run.

The high-energy bench man sparked this series of possessions: Pietrus hit a three-pointer after a loose ball rebound; the Sixers missed a jumper; Pietrus hit another trey; Pietrus got a block on the defensive end. It was a six-point swing. More important in this league of runs, though, was the momentum swing.

Pietrus came out of the game with seven minutes to play in the fourth quarter and didn't see the court again, turning it over to Boston's usually trustworthy Big Four.

But Evan Turner hit two free throws with 12 seconds remaining to give the Sixers a 78-75 lead that they did not relinquish.

Garnett was called for an offensive foul with 10 seconds remaining, and Lou Williams hit both ensuing free throws to ice the game despite a successful Allen three-ball with 2.2 seconds remaining.

"I think Kevin got three off-the-ball offensive fouls," Rivers said. "So it looked like [the officials] were looking for it all night."

Rajon Rondo, the human stat sheet, did all he could by distributing two-handed heaves. But the Celtics couldn't finish shooting from the floor. He ended with 13 assists and could have added 10 more had Boston not gone ice cold from the floor. The Celtics finished shooting 43 percent.

Brandon Bass hit his first three shots of the game and gave the Celtics an early 9-0 lead. Then he struggled from the floor and finished 5 of 15 shooting. Pierce, who is hampered by a sprained ligament in his knee, continued sending bricks at the rim. He finished 2 of 9 from the field for seven points.