Watch Hawks center Onyeka Okongwu with game-winning tip-in to beat Celtics.

Trae Young was out with right Achilles tendonitis. Bogdan Bogdanovic was out with right hamstring tendinopathy.

But the Atlanta Hawks had Onyeka Okongwu, and the big man was there when they needed him most, tipping a game-winner.

Boston had a couple of chances to get the win back, but had a turnover on an inbounds pass to Jayson Tatum, then Jaylen Brown missed a good look at mid-range, and the Hawks escaped with a win in Boston.

Dyson Daniels stepped up on both ends of the court, scoring 28 to lead the Hawks, while veteran Larry Nance Jr. added 19 off the bench for Atlanta. Okongwu finished with 15 points.

Brown had been on fire for most of the game and finished with 37 for Boston, while Derrick White had 31 points including seven 3-pointers.

The Celtics haven’t been sharp lately — they have enough talent to keep winning games even without Kristaps Porzingis (and Brown missed a handful of games as well), but they haven’t looked like themselves. Call it a mini-slump, a delayed championship hangover, or whatever you will, but it was a rough loss.

Atlanta is now 1-0 in NBA Cup games for Group C. For Boston to advance from the group stage to the knockout round, it must beat Cleveland in a massive game next Tuesday, win out to go 3-1 and have a a couple of blowouts in there (point difference is a tiebreaker).

To do all of this, the Celtics will need to be sharper than they have been recently.