BOSTON -- It's one thing for Paul Pierce to miss shots. He's done plenty of that while laboring through a five-game shooting slump. But to flat-out pass up open looks? That's proof that Pierce just isn't himself at the moment.

Pierce scored a season-high 19 first-half points on Thursday night against the New York Knicks, but put up only three second-half shots -- missing them all -- while scoring a mere three points after the intermission as the Boston Celtics fell 89-86 at TD Garden.

Pierce, battling a terrible shooting slump, connected on 6 of 15 shots overall, but it wasn't the fact that he missed nine attempts; it was the fact that he passed up open looks in key moments and then let the game slip away with a turnover on the Celtics' final possession as the Knicks escaped with their first win in Boston since 2006.

With the Celtics down 84-82 with little more than two minutes to go, a driving Avery Bradley kicked to Pierce at the top of the key. Pierce, 0-for-12 over the past five games from beyond the top of the arc, elected to get a charging Jason Kidd in the air with a pump fake, then stepped up to his elbow sweet spot. Even with a triangle of defenders including Carmelo Anthony and J.R. Smith in front of him, Pierce had space to get up a shot. Instead, he kicked to Avery Bradley, who missed a corner 3 that would have given Boston the lead.

Paul Pierce came unglued with the game on the line. Jared Wickerham/Getty Images

"I'm taking the Paul shot," Celtics coach Doc Rivers said. "Nothing against Avery, but Paul Pierce wide open at the elbow? He turned into Kevin Garnett for one play, where you want him to shoot and he makes the pass. You can't fault him for moving the ball."

Yes, that was somewhat excusable. What happened soon after wasn't quite so easy to swallow.

With New York out front 89-84 with 66 seconds to play, Pierce inbounded to Jason Terry, then circled back out to take a handoff. Improbably, the ball popped loose and tumbled out of bounds.

Boston's defense got it back and, after a Rondo jumper made it a one-possession game again at 89-86, Boston got a chance to tie with 13.1 seconds remaining. This time Pierce tried to cut from the left wing, but got snagged a bit on J.R. Smith. A Rondo feed ticked off his hands as Pierce couldn't get the handle, eventually kicking it out of bounds in front of the Knicks' bench.

New York ran out the clock from there, snapping a 13-game losing streak (regular season and playoffs) at TD Garden. The Knicks now own a seven-game lead over Boston in the Atlantic Division.

"Down the stretch, we had poor execution. Simple and plain," Pierce said. "You know when you need buckets, when the game is tight, you can't turn the ball over. You know and that's what we did down the stretch."

Honing in on the final offensive sequence, Pierce noted, "The play was broken up and I tried to flash to the ball. I really didn't get a good hand on it and J.R. made a good stab at the ball and it resulted in a turnover."