Rajon Rondo chases down the ball as time expires in Game 5 of the Boston Celtics series against the Atlanta Hawks. (TAMI CHAPPELL / Reuters)

Doc Rivers wasn’t happy with the way the Boston Celtics played late in Game 5 of their Eastern Conference semifinal series against the Atlanta Hawks.

And he had a couple of awful moments to think about, moments that caused his team to lose 87-86. Instead of wrapping up the series and giving Paul Pierce a little break during which he could rest the knee he hurt in a Sunday morning shootaround, the Celtics now face a Game 6 on Thursday night.

First, Pierce missed a 20-foot jumper with 18.9 seconds left and the Celtics were sluggish in trying to foul the Hawks.

“We signaled to foul, but our guys decided to go for a steal first. What they didn’t understand was that we had a foul to give. I told them after the game — we talk about it every night and every day in practice — end-of-the-game execution is going to win or lose playoff games for you,” Rivers said. “You think about it, that’s seven extra seconds that [Rajon] Rondo would have had [for a final-seconds shot, which he could have used].”

Then, the Hawks nearly handed the game to the Celtics, who handed it right back. Rondo intercepted an inbounds pass with about nine seconds left and quickly moved up the floor, since the team had no timeouts. But Rondo couldn’t move or get off a shot and time ran out. Rivers liked Rondo’s defensive play — “a terrific defensive play” — but “we were in a jumble. I wish [Rondo] went down the middle of the floor, I think you should always go down the middle of the floor, especially when have a big [guy] on you,” Rivers said. “Listen, it’s a turnover, it’s a fast break — we were just trying to go.”

Now, it’s back to Boston for Game 6. No Celtics team has lost a series when leading 3-1 and, really, are they going to lose three in a row to the Hawks? Still, there’s also no mistaking the sting of an opportunity frittered away. “They’ve got confidence and momentum,’’ Pierce with a sigh. “We have to go home and get the job done.”

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