"We need to come out and destroy teams early," Stephenson said. "We can't rely on the second half to win the game."
Jeff Green scored 20 for the Celtics, who lost their sixth straight to drop to 4-10 after winning four in a row.
The rebuilding Celtics played impressively in the first half against a team that had won five of its first 11 games by at least 10 points. But Boston had more turnovers, nine, than points in the third period. And West outscored Boston's entire team in the quarter with 10 points.
"Our guys' energy level, enthusiasm and intensity was really the difference" in the second half," West said.
The Pacers took their first lead since 7-6 on a tip-in by Roy Hibbert that made it 55-54 with 5:48 left in the third quarter. Green's two free throws gave the Celtics the lead again. But they never got it back after George's 3-pointer with 4:05 left in the period began a 10-0 run that gave Indiana a 65-56 lead.
Stephenson followed with a layup, George hit a 20-footer, West made a short bank shot and Ian Mahinmi sank a free throw.
The surge began after Crawford missed for the first time on a jumper from the left corner with 4:15 remaining in the third. He had made his first nine shots and both free throws.
"We didn't play with the same intensity that we had in the first half," Boston's Jared Sullinger said.
The Celtics had taken their biggest lead of the game, eight points, on Crawford's 30-foot 3-pointer at the halftime buzzer.
Boston sank 59 percent of its shots in the half, while Indiana hit 44.4 percent. But in the third quarter, the Celtics hit 21.4 percent (3 of 14) to 42.3 percent (11 for 26) for the Pacers.
Indiana had won its first nine games before struggling as it began a three-game road trip. It lost to Chicago 110-94 then went to overtime before beating the New York Knicks 103-96 Wednesday night.