BOSTON — Jaylen Brown As he sat down for his postgame press conference Tuesday night, he could only shake his head after seeing the box score. Celtics poised to win its third game in a row We took a 15-point lead in the second half against the Hawks and watched him slip away in the second half after a series of efforts and mental miscues.

The box score numbers were eye-popping on that front. The Hawks took a 68-32 lead in the paint thanks in large part to 20 offensive rebounds. Those rebounds and 20 Celtics turnovers led to 25 extra field goal attempts for the Hawks, paving the way for an undermanned Hawks team to take a 117-116 victory in front of a stunned TD Garden.

It was a rare disappointment for a Celtics team that prided itself on avoiding such ugly losses to lower-level opponents last year. Despite scoring a game-high 37 points, Brown was unhappy with his performance.

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“I thought we were careless mentally tonight,” Brown said. “We were expecting to win. He was very careless and it showed. I think the No. 1 category it showed was our turnovers. In any case, having 20 turnovers is not good for us. I think it showed that we weren’t in sync like we normally are, so we’ve got to be better at that and I’ve got to be better at it.” I need to be.”

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

“I still give credit to Atlanta,” Brown said. ‘They came out and played physical. But me and JT need to get better. As a team, we had 20 turnovers, he and I had 11 total turnovers. Some of them were offensive fouls or something. But we still need to secure the ball better and keep our distance. We couldn’t do that tonight and it cost our team.”

After starting the year with a 7-1 record, the Celtics are 2-2 in their last four games and remain three games behind the undefeated Cleveland Cavaliers at the top of the East. A bad loss doesn’t define a team, but the Celtics looked pretty lethal last week after getting off to poor starts against the Bucks and Nets.

“It’s just a mentality,” Brown said of the struggles. “This is part of the journey. We have some things to clean up. We are not a perfect team. This is a new season, a new journey and we are looking forward to embracing these moments. We will monitor it and address what needs to be addressed.

“We need to be able to move forward. We can’t let this become a habit. We need to be able to respond well. That’s why I’m looking forward to the movie. We have back to back work ahead of us, we will put this behind us. But we will learn from these situations and get better.”

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