Williams scored a season-high 25 points in his return to the lineup, Brook Lopez added 24, and the Brooklyn Nets beat Boston 104-96 Tuesday night in the first matchup for Pierce and Garnett against their former team.
"D-Will set the tone for how we were going to play tonight," Garnett said.
Garnett had 11 points, just his third time in double figures this season, and grabbed nine rebounds. Pierce finished with four points and seven rebounds in his first appearance off the bench in six years after making a quick recovery from a broken right hand.
But the key was Williams, who had missed nine straight games and 11 overall with a sprained left ankle, and hadn't been sharp in the games in which he did appear.
"He's the head of the snake," Pierce said. "I mean he's one of the most important pieces of what we're trying to do here, what we're trying to build and trying to win a championship."
The Nets won consecutive games for the first time this season, a stunning stat for a team that considered itself a championship contender after acquiring Pierce and Garnett in a blockbuster that was agreed to on draft night.
Both veterans had struggled, with the 12.4 points per game and .368 shooting by Pierce easily the lowest of his career before he was hurt on Nov. 29. The Nets had lost six straight at home, their longest skid since moving to Brooklyn.
"We're starting to get whole and it's definitely good to have everybody back," Williams said. "Even though Paul didn't start, I thought he came in and gave us a big boost off the bench. It seemed like our energy level was a lot better. More intensity on both ends of the floor."
Avery Bradley scored 22 points and Jeff Green had 19 for the Celtics, who have been competitive after trading away two of their most important players from their 2008 NBA championship team. Boston had a three-game winning streak snapped and its Atlantic Division-leading record fell to 10-13.
"The players that they had come back tonight, I still feel like they were a very good team. But obviously having two of their leaders as in Deron Williams and Paul, you know that definitely helps them out a lot," Bradley said.
The Celtics had started with 14-0 and 12-0 leads in their last two games, the latter eventually ballooning to a 45-point bulge in a 114-73 victory over the New York Knicks on Sunday that was the NBA's most lopsided game this season.
Brooklyn got off to the strong start in this one, pulled away again after Boston tied it, and put it away when Williams, who came in averaging just 9.3 points, scored eight in the final period.
"They played great tonight. Played really hard, really together. They took us out of what we wanted to do," Celtics coach Brad Stevens said. "Like I said, we weren't playing the New Jersey Nets that played without D-Will for those 10 games. That is their team and they're a good basketball team."
Pierce was expected to miss two to four weeks, but sat out only four games before returning to practice Monday. He received a nice ovation when he checked in with a protective glove covering his hand midway through the first quarter, no doubt aided by the many Boston fans that were noticeable all around Barclays Center.
He said he's still getting used to the glove, joking he might take it out to the movies, and might have to get used to a new role. Coach Jason Kidd said he would consider whether Pierce would remain with the second unit.
Garnett gave a pat on the head and a little hug before the opening tip to Boston's starters, who go on to face former coach Doc Rivers and the Clippers on Wednesday.
The Nets led 31-20 after one, their largest lead after the opening quarter this season, and 56-47 at the half.
Boston quickly tied it at 60, but the Nets responded with 10 straight points, going ahead 70-60 on Williams' jumper, and the lead reached 85-68 on Pierce's free throw with 1:14 left.
It was the first time Garnett played against the Celtics since March 4, 2007, for Minnesota. ... Boxer Paulie Malignaggi was presented with the championship of Brooklyn belt by Barclays Center CEO Brett Yormark in the second quarter after beating fellow Brooklyn product Zab Judah in a welterweight bout here Saturday.