On their biggest defensive sequence of the game to that point, they allowed an uncontested layup. They refused to protect the edge. They made silly turnovers with apathetic passing that was downright disrespectful to the Atlanta defense.

And when they desperately needed a bucket in the last 40 seconds, they had nothing left. They lost inexcusably 117-116 to the small Hawks after blowing a 15-point lead in the third quarter.

The Hawks were without All-Star guard Trae Young, yet the Celtics were outplayed, outmaneuvered and outcoached by an inferior opponent.

The Celtics’ chances of reaching the quarterfinals of the NBA Cup are now so damaged that they will likely have to win their last three games to have a chance of reaching the quarterfinals, including a matchup against the undefeated Cavaliers.

And they have finally paid for the defensive slippage, lack of rim protection and half-hearted play that has plagued them over the past two weeks. Boston scored 20 goals, yielded 20 offensive rebounds and 38 Atlanta points in the second half.

The Hawks used their size and physicality to exploit the Celtics’ softness up the middle. Rim protection and defense becomes a problem. The Hawks inside duo of Clint Capela and Onyeka Okongwu were a combined 15-of-22 shooting with 10 offensive rebounds.

They bullied any Celtic brave enough to stand in their path, or they crossed and simply shot over smaller players.

The Celtics may be off to a respectable 9-3 start, but they are a team with issues to address. They need to be stronger, they need to rebound better and they need to hit first.

Once again, they started this game slowly, only to find a way to take control, then have their arrogance disappear once they had a 15-point lead midway through the third quarter.

The Celtics thought the upstart Hawks were done, but Atlanta kept pushing, kept attacking the paint without resistance and then gained more confidence. The Celtics’ approach in the second half was almost disrespectful to their opponent.

After the game, Celtics coach Joe Mazzulla was briefly but not exactly irate. In his own special way, he seemed to accept that his team is far from a finished product, that they have glaring weaknesses, and that the embarrassing loss might just be a reason to re-emphasize the little things, especially the robustness.

“They beat us in every facet of the game and beat us on all (statistical) margins. They deserved to win the match,” said Mazzulla. “They played harder and more physical than us. They got 20 offensive rebounds and forced us into 20 turnovers.”

The lesson here is that the Celtics are beatable, especially if they’re not making threes at a 50 percent clip. Mazzulla likes to point out the “margins,” meaning shot attempts, three-point attempts, assists and turnovers. The Hawks won because they attempted 25 more shots, thanks to those offensive rebounds and Celtics turnovers.

The Celtics rank last in the NBA in points in the paint, as they lack a dominant post presence and 56.5 percent of their shots are three-pointers. But they rank 23rd in the NBA in paint points allowed; This means teams are breaking down their defense by twos every night.

And the Celtics need to make those two more difficult. If not, they rely primarily on three-point shooting to win.

“They got rebounds and layups on everyone, big, small, medium,” Mazzulla said. “They just beat us on both ends of the floor.”

When asked if the Celtics need to improve their rim protection, Mazzulla said: “Absolutely.”

After the match it was quiet in the locker room. Jaylen Brown and Jayson Tatum know they both have to be better. Brown scored 37 points but missed three key free throws in the fourth quarter and committed six turnovers. Tatum still doesn’t seem 100 percent after spraining his ankle on that no-call Sunday in Milwaukee, then readjusting on a blocked shot by Larry Nance Jr. with 4:29 to go.

He worked after that play and went without a field goal in the fourth quarter.

“It’s already difficult to win in this competition. And if you give them second and third chances and you turn the ball over instead of getting a shot up there, it makes it harder for you,” Tatum said. “We let go of the rope at the end of the third and gave them lights and we just made it a ball game.”

There will be no rest. The Celtics will play again on Wednesday against the same Brooklyn team that pushed them to overtime on Friday by playing harder and wanting more. The Celtics need to regain their toughness. They need to be more physical and imposing. They need to start diving for loose balls again and adjust their opponent’s intensity.

They need to drop the arrogance and play with the same desire and passion as last season.

“Mentally we were too careless,” Brown said. “It showed that we weren’t in sync like we normally were, so we have to be better at that. We didn’t fight like we normally do. This is part of the journey, but we still have some work to do.

“We are not a perfect team. It’s a new season, a new journey, and we look forward to embracing those moments.”

Gary Washburn is a Globe columnist. He can be reached at [email protected]. Follow him @GwashburnGlobe.