Powe had his 10th career double-double and second straight since replacing Davis, who was injured while plugging the more sizable hole opened when Garnett went out with a right knee sprain.
"Since he's been put in a starting role, he's been really playing well for us," said Celtics swingman Paul Pierce, who scored 17. "That's what we need now, especially with guys injured. Hopefully this will carry over when Kevin gets back."
Rudy Gay scored 26, O.J. Mayo had 21 and Marc Gasol had 11 points and 15 rebounds for the Grizzlies, who have lost four consecutive games as they struggle to stay out of the Western Conference cellar.
Garnett missed his 10th game in a row since injuring his knee against Utah on Feb. 19, and Boston is 6-4 without him to fall 2 1/2 games behind the Cleveland Cavaliers in the race for home-court advantage in the Eastern Conference playoffs. Davis had been starting at power forward until he was hurt against Orlando on Sunday.
That put Powe in the starting lineup for the 10th time in three seasons.
"He was just an animal. Aggressive, energetic, live-wire body," Grizzles coach Lionel Hollins said. "They played the way a defending champion plays against a team they thing they should beat. They just stayed ahead."
Boston has also been without Brian Scalabrine for most of February and March with concussions, and Tony Allen has played in eight games since New Year's Day because of a left thumb injury. The Celtics did get some good news when point guard Rajon Rondo returned after missing two straight games with a sprained right ankle.
"When we're at full strength, we'll be tough to stop," Powe said. "I want KG to come back. We miss him."
But even against the Celtics' second- and third-stringers, the struggling Grizzles put up little fight.
Boston trailed by seven early in the second quarter before scoring 11 straight points -- most with the backups on the court -- to take the lead. Mayo hit three straight baskets in the third quarter, including a 3-pointer with 7:55 left, to give Memphis a 59-57 lead.
But the Celtics scored the next eight points and opened a 10-point lead before the end of the third.
Stephon Marbury had his best game since joining the Celtics, scoring nine points with five assists. Marbury scored eight in his Celtics debut, then had six points combined in the next five games.
The Celtics wore special uniforms, green with gold trim, in honor of St. Patrick's Day. ... Fifteen of Memphis' last 19 games are against teams with winning records. ... Boston used its eighth starting lineup of the season. ... Mikki Moore picked up four fouls in his first seven minutes on the floor. ... Powe's double-double is the 10th of his career.