Running the floor and looking strong on the glass, the 35-year-old Garnett matched his season-high with 24 points and grabbed nine rebounds to lead the Celtics to their fourth straight win, a 98-80 victory over the Memphis Grizzlies on Sunday.
"He was feeling good. It's a still a kids' game. He was having fun," Rivers said of Garnett, a 14-time All-Star. "He was having a ball playing today and you don't want to tell him you have to come in because the lights are on, so we let him play in the dark a little bit."
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Garnett was 9 of 12 from the field, including his third 3-pointer of the season on three tries. He hit one in Friday's win over New York.
"He's shooting it well," Rivers said, before joking about Garnett's recent 3-point success. "Gives us another option at the end of the game - don't tell him that."
Paul Pierce scored 21 points, moving nine behind Hall of Famer Larry Bird for second place on the club's all-time list, and Ray Allen added 12. Rajon Rondo had 14 assists and reserve Chris Wilcox chipped in with 12 points.
Boston took charge with a 21-7 run early in the fourth quarter that turned a six-point lead to 90-70 on Garnett's jumper.
Relying on a combination of reserves and two of their usual stars -- Garnett and Pierce -- the Celtics controlled the glass and outran the Grizzlies for transition baskets.
"I thought we put together some really good runs and got the ball moving," Pierce said.
Rudy Gay paced the Grizzlies with 21 points and seven rebounds, and O.J. Mayo scored 15 points for Memphis, which has lost six of eight.
The Celtics led by six points at halftime and pushed it to double digits (53-43) on Pierce's two free throws midway into the third. Boston relied on some hot long-range shooting in the third quarter, hitting 5 of its first 6 from behind the 3-point line to keep Memphis from coming back until Gay scored three consecutive baskets late in the period.
"The key to the game was that we didn't get back on defense. They had 26 fast break points on us and that was huge, and they controlled the glass," Grizzlies coach Lionel Hollins said. "We turned them over but we couldn't get the easy transition buckets. They got back on defense and forced us to play half court, and they did a good job defensively."
Allen hit three shots from beyond the arc and was fouled on one, hitting the free throw that made it 62-51. Pierce and Garnett also hit 3s, with Garnett's bringing a loud ovation from the crowd.
"I can shoot 3s," Garnett said. "Everyone's acting like I'm 50 and out here on one leg. I don't shoot 3s because we have one of the all-time greatest 3-point shooters in the history of the league [Allen], we have Paul Pierce who has won 3-point contests, we have other guys who can shoot 3. That ain't my role here. My role is to get those guys open and shoot it, dunk it, pass it, whatever to get us going. But, I can shoot 3s."
The Grizzlies knew they were in trouble.
"They got on a run right before the end of the quarter," Memphis guard Mike Conley said. "Ray Allen hit that corner 3 and we couldn't recover."
The Celtics held a 69-61 lead entering the final quarter.
Boston led 43-37 at halftime after the teams traded large second-quarter runs. The Grizzlies opened the period by scoring 16 of the first 19 points, taking a 32-27 lead on Dante Cunningham's jumper.
After Rivers called time out, his team responded with a 16-2 run -- highlighted by a 7-0 spurt that was capped by a 3 by Allen from the left corner and Jermaine O'Neal coming down the lane for a dunk after a nice feed from Rondo, who had 10 assists in the opening half.
- Garnett also had 24 points in a New Year's Day win over Washington.
- O'Neal went to the locker room briefly late in the third quarter.
- Memphis guard and former Celtic Tony Allen didn't play because of a sore hip and knee, but he was in uniform on the bench.
- Rivers said F Brandon Bass was out with "swelling of the knee." Rivers didn't make it sound like it was serious. "I think it's an accumulation of a lot of games," he said.
- The game started just after noon. "I don't like 'em," Rivers said of the earlier start. "I like them on the road. I think it favors the road team. You get on the bus and get ready."
- Bird had 21,791 points. Hall of Famer John Havlicek tops the Celtics' all-time scoring list with 26,395.
- Rondo played his second straight game after missing eight with a sore right wrist. He wasn't wearing a brace like he did in Friday's win over New York.
- Gay came down awkwardly after hitting a jumper and limped for a few seconds, but stayed in the game.