Boston (40-9), which finished last season with a 24-58 record, plans to save any celebrations for the postseason.
"There's a long way to go," Celtics coach Doc Rivers said. "We're not looking at numbers or anything like that. We're looking at getting better and getting ready for the playoffs."
Paul Pierce, one of the few holdovers from last year's team, said reaching 40 wins shows the Celtics are heading in the right direction. Boston has won four straight and six of seven.
"Right now, we're just trying to build good habits," he said. "Each win is great, but we're building good habits."
Pierce finished with 28 points and 12 rebounds, Ray Allen scored 23 and Leon Powe added 16 points and nine rebounds for the Celtics, who have been without Garnett (abdominal strain) for the past eight games and Perkins (strained left shoulder) the past three.
"That's no excuse for us," Pierce said. "Until those guys get back, we've got to hold down the fort."
"The signs are there that we're getting better," Williams said, "but staying with good teams and beating them is two different things."
Trailing by five points entering the fourth quarter, the Pacers took an 86-84 lead on Williams' 3-pointer with 9 minutes left. Boston answered by scoring the next nine points, including Allen's 3 to cap the run, to open a seven-point lead.
Indiana cut the deficit to 95-94 following Granger's 3-pointer and Williams' layup with 3 minutes remaining, but the Celtics regained a 101-95 lead on Pierce's jumper with 52 seconds left.
Rivers was impressed with the Pacers' ability to keep the game close.
"We couldn't shake them," he said. "They played hard throughout and made some big shots."
Indiana coach Jim O'Brien was more critical of his team.
"A couple things in the fourth quarter really hurt us -- turnovers and we missed a couple of shots right at the basket that could have turned the tide," O'Brien said.
Allen scored 10 of the Celtics' first 12 points of the game, including a 3-pointer to give Boston a 10-0 lead. The Pacers fought back, taking a 13-12 lead on Troy Murphy's two free throws later in the period.
Indiana opened its biggest lead of the game 34-23 on Marquis Daniels' basket early in the second quarter, but the Celtics regained a 49-45 lead following Rajon Rondo's 3-pointer with less than 2 minutes left in the half.
Rondo finished with 12 points, seven assists and seven rebounds.
"He's growing up into one of the premiere guards in a matter of one year," Pierce said. "His game is stepping up by leaps and bounds throughout the course of the season."
In the third quarter, Boston extended its lead to 75-65 on Allen's jumper with 3 minutes left. The Pacers got within 82-77 after layups from Kareem Rush and Murphy to end the period.
"This was definitely a winnable game," Granger said. "We let it get away at the end with a lack of execution and our usual letdown in the stretch."
O'Neal missed his 12th straight game for the Pacers with a sore left knee, and Tinsley also missed the game with a sore left knee. ... Granger extended his streak of consecutive free throws made to 56. He finished the game 5-for-5 from the line. ... The Celtics' only loss during their past seven games came Feb. 5 at Cleveland. ... The Pacers, who play at Detroit on Wednesday, dropped to 3-13 in the first game of back-to-backs this season.