BOSTON — Jaylen Brown could only shake his head after spotting the score as he sat down for his post-match press conference Tuesday night. THE Celtics positioned themselves to win their third straight match by take a 15-point lead in the second half against the Hawks and saw him escape in the second half behind a multitude of efforts and mental errors.

The numbers in the score were glaring on that front. The Hawks dominated scoring in the paint by a 68-32 margin, thanks in large part to 20 offensive rebounds. Those rebounds along with 20 Celtics turnovers led to 25 more shot attempts for the Hawks, opening the door for an undermanned Hawks team to take the 117-116 victory in front of a stunned TD Garden.

It was a rare disappointment for a Celtics team that prided itself on avoiding those kinds of ugly losses last year against inferior competition. Brown was unhappy with the performance despite scoring a game-high 37 points.

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“I think mentally we were sloppy tonight,” Brown said. “We expected to win. Too careless and it showed. I think the No. 1 category he showed up in was our turnovers. It’s not typical for us to have 20 turnovers under any circumstance. I think it just shows that we weren’t in sync like we usually are, so we need to be better at this, and I need to be better at this.

The Celtics dominated this same Atlanta team two weeks ago on the road, but this time around they brought more energy even without star guard Trae Young. Yet the Celtics’ most glaring mistakes were those of Brown and Jayson Tatum who combined for nine of the team’s 20 turnovers.

“I always give Atlanta credit,” Brown said. “They came out and played physical. But me and JT need to get better. We had 20 turnovers as a team and he and I both had 11 total. Some of them were offensive fouls and such. But still, we just need to secure the ball better and achieve our spacing. And we didn’t do it tonight and it cost our team dearly.

The Celtics are now 2-2 in their last four games after starting the year 7-1 and sit three games behind the undefeated Cleveland Cavaliers atop the East. One bad loss doesn’t define a team, but the Celtics looked very lethal last week amid poor starts against the Bucks and Nets.

“It’s just a mentality,” Brown said of the struggles. “It’s part of the journey. We have stuff to clean up. We are not a perfect team. It’s a new season, a new journey, and we can’t wait to experience these moments. We’re going to monitor it and we’re going to address what needs to be addressed.

“We have to be able to move forward. We cannot let this become a habit. You have to be able to react well. So I can’t wait to film. We have a back-to-back game coming up, we’re going to put it behind us. But we will learn from these situations and we will be better.

The Celtics will need to quickly turn things around Wednesday night when they face a rested Nets team at Barclays Center.