BOSTON — The New York Knicks must figure out how to finish off games.

Just watch the Boston Celtics.

The Celtics led for just 60 seconds in the second half until Ray Allen sank a go-ahead 3-pointer with 12 seconds left and beat the Knicks 87-85 in their playoff opener on Sunday night.

“Down the stretch we found a way to win,” Paul Pierce said, “and that was because of our experience.”

The Celtics won their NBA-high 17th championship in 2008 and lost in Game 7 of the finals last year to the Los Angeles Lakers. The Knicks, in the playoffs for the first time in seven seasons, last won a postseason game on Apr. 29, 2001.

“We can compete with them, obviously,” New York coach Mike D’Antoni said. “They made big shots at the end. That’s why they are who they are.”

The Knicks ability to make those clutch shots may have suffered a blow when Chauncey Billups left with 51 seconds remaining with what he said was a strain in his left knee or thigh muscle.

He and D’Antoni said they didn’t know his status for Game 2 of the best-of-seven Eastern Conference first-round series on Tuesday night.

On Sunday, it was Toney Douglas who hit a 3-pointer with 39 seconds left that put the Knicks ahead 85-82. But after a timeout, the Celtics scored just one second later on an alley-oop pass from Rajon Rondo – who had 10 points, 9 rebounds and 9 assists – to Kevin Garnett.