"With so many guys out, we just have to try and maintain," said Artest, who scored 22 points after halftime. "That way when they come back, we'll be that much stronger."
O'Neal, who missed Wednesday's season opener at Cleveland, entered the game midway through the first quarter. He shot 5-of-16 from the field and 9-of-14 from the free-throw line.
"Right now I don't deserve to be a starter because I haven't been there," said O'Neal, who played in just one preseason game and was limited to 24 minutes on Friday. "I've got to work my way back in the lineup. I truthfully believe I don't deserve to start yet."
The Pacers have won their first two games of the season for the first time in nine years, while Boston is 0-2 under new coach Doc Rivers.
"This is getting old already and it's only been two games," said Rivers, who has watched his team squander a late lead in both games. "We've just got to get tougher."
Paul Pierce, guarded by Artest for most of the game, was held to 15 points on 5-of-14 shooting. Pierce had 35 in a season-opening loss to Philadelphia on Wednesday night.
The Celtics led for much of the game, but Indiana took the lead for good with 4:55 remaining in the fourth quarter on Austin Croshere's 3-pointer. Croshere's layup with 57 seconds left capped a 15-4 spurt.
The Celtics were outrebounded 52-31, including 13-3 on the
offensive end. Indiana guards Fred Jones and Jamaal Tinsley each had eight rebounds.
"We have been playing great in the first three quarters and then slack up in the fourth quarter," Payton said. "I think it's just mental. We need to be mentally tough. There are a lot of young guys in here and I think we stop playing in the fourth quarter."
Croshere added 16 points for the Pacers.
Pierce's jumper with 1:45 left in the second gave the Celtics their biggest lead of the first half at 50-43. Indiana responded with an 8-2 run, capped by Stephen Jackson's shot with 1.4 seconds left, to pull within one at the half.
Bender sat out with a sore right knee after scoring 11 points on Wednesday . ... Celtics rookie Al Jefferson scored his first career points on a layup with 3:25 remaining in the first half. ... Payton is 24 points shy of 20,000 career NBA points. He'll be the 29th player in NBA history to reach that milestone.