After what looked like a listless first half, Boston took over with an 18-3 third-quarter closing run.
"He mainly talked about us," Pierce said Rivers told the team after the sluggish first half. "He just told us we should win this game with all the injuries they have. We just needed to clean up some things and stop turning the ball over."
Reserve Brandon Bass scored 15 and grabbed 13 rebounds, and Rajon Rondo had eight points and 12 assists for the Celtics (4-3). Boston has won four straight since dropping all three of a season-opening road trip.
Kevin Garnett had 14 points and 12 boards, and Bradley finished with 11 points.
"We just played better, played harder, executed better," Rivers said of the change after halftime. "I really thought in the first half our defense was terrific; our offense was horrendous."
Rivers also said Pierce's play is improving after he missed the initial three games with a bruised right heel.
"You can see it," he said. "He's getting his rhythm."
MarShon Brooks, making his first career start, led the Nets with 17 points, and Jordan Farmar had 11. New Jersey has lost six straight since a season-opening win at Washington.
"I thought it was going to be a big game until it happened," Brooks said. "Boston did a good job of adjusting at halftime and throwing some double teams at me. It was my first time getting double teamed in this league, so I didn't know how to get my shot off."
Nets guard Deron Williams missed the game with a sore right ankle, forward Kris Humphries with a sore left shoulder and center Brook Lopez with a broken right foot. Further depleting their roster, starting forward Damion James had his right foot stepped on late in the second quarter and didn't play the second half.
Brooks had 15 in the first half, but was hobbled after tweaking his left ankle late in the second quarter.
"He had an awfully good first half for us. He showed how explosive he is offensively and I think since he kind of tweaked it in the first and it took a lot of his energy in the second half," Nets coach Avery Johnson said. "He tried to play through it -- he had a really nice first half for us, as well as our team, defensively. We just couldn't score in the second half."
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The Nets are hoping Brooks and James aren't out now, too. Brooks had X-rays in the arena after the game, but results were not immediately available to the media.
"Damion is more of a foot situation and MarShon is more of an ankle," Johnson said. "We'll have more of an update on them tomorrow."
Boston guard Ray Allen missed the game due to illness.
Neither team shot over 40 percent in the first half that ended with the Nets up 35-34. To add to its weak shooting first half, Boston had 10 turnovers.
But in the third quarter, the Celtics just had too much offense -- mainly Pierce.
"We just started to execute and run a little bit," Pierce said. "We kept it simple."
Bass started the spree with a jumper from the wing and Rondo hit a free throw before Pierce nailed consecutive 3s just 34 seconds apart, making it 55-44.
He then hit one free throw before Bradley, who played sparingly in just his second season, nailed his first 3 of his brief NBA career and Pierce hit two more free throws, making it 61-46.
Bass' jumper late in the quarter gave Boston a 63-47 lead after three quarters.
Rondo's jumper midway into the final quarter gave the Celtics a 73-51 lead.
Brooks became the Nets' fourth rookie since 2000-01 to start within the first seven games of a season. ... Boston C Jermaine O'Neal returned to the starting lineup after missing Monday's game with a tight right hamstring. ... Patriots NT Vince Wilfork was seated courtside. ... Bradley is now 1 for 11 in his career on 3-point attempts.