The Celtics' first loss of the season revealed some malaise at the start of the season

Their overall talent is good enough to win every night, and there will be rare occasions this season where the Celtics aren't favored. But Tatum and Brown attributed Boston's 24-point deficit in the third quarter to a lack of energy.

They certainly didn't lack energy in Monday's 119-108 road win over the Bucks, and they rebounded by using Tuesday as a travel day. But there will be nights during the grueling NBA season when legs are heavy and the opposing team has more motivation, especially the Pacers, who have become a new rival and have suffered three straight losses.

Maybe Indiana wanted it more on Wednesday. And there will be nights when the desire is missing. But the Celtics won't be fooled by their ability to rally, play ten brilliant minutes and turn the Pacers into a bumbling bunch in the final minutes.

Tatum and Brown combined to go 6 for 29 from the 3-point line and neither was good except in a few key moments. The Celtics deserved to lose because they were outplayed and outsmarted, and those nights are going to happen.

The good news is that everyone has taken responsibility for their efforts.

“I just didn’t like the way I started the game,” Tatum said. “I set the tone negatively and my energy wasn't where it needed to be and I just felt like it had a domino effect. We were a step behind on offense and defense, so I take a lot of the blame for that. I just wanted to keep going in the second half.”

The Celtics have been hearing how good they are for weeks. And they mostly played like that, a dominant start against the Knicks, an easy win against the Wizards. But they needed a late comeback to pull out a win in Detroit and then a dominant fourth quarter to beat the Bucks.

Against the Pacers, who led for two and a half straight quarters behind Bennedict Mathurin and Siakam, they were never in control before Tatum tied the game with a 3-pointer with 14.4 remaining in regulation. The Pacers got every loose ball and every important offensive rebound, while the Celtics wasted numerous chances to recover.

On her last offensive possession, Brown hit a potential 3-pointer with 5.3 seconds left. It was a fitting end to a difficult night for Brown, who dominated the Eastern Conference finals series against Indiana just five months ago.

“Our energy was lacking and a lot of that is on me,” Brown said. “I have to be better for my boys. Just converting at the edge, simple things, making open shots. We got ourselves into a rut early in the game because we just didn't convert open looks, weren't physical enough on drives, finished at the rim, and a lot of that is my fault. But it is a way to defend yourself. We had a chance to win.”

The sign of a great team is to never give in, even when they have every reason to. Celtics coach Joe Mazzulla, who prides himself on the unconventional, decided to stick with his starters along with reserve center Neemias Queta as the Celtics fell 112-91 in the final nine minutes.

That quartet provided a momentum, and then Mazzulla got Tatum to join in, sparking a 33-12 run at the end of the fourth quarter. The turnover was a sign of greatness, but the Celtics can't rely on those stretches to save them, even though they will on most nights.

“It’s a luxury, we have a good group, we have a good unit,” Brown said. “But we can’t take games for granted. We had a chance to win tonight, but we didn’t do it.”

Tatum and Brown know they didn't play anywhere near their level and that's why the Celtics lost. But they also saw how quickly they could flip the switch. Wednesday was a valuable lesson in a season that offers constant learning opportunities, even for such an experienced team.

“It was a story of spurts,” Tatum said. “When we played Celtics basketball, it showed that we don't turn over, we come back in transition and get stops and go out and run and play to our strengths, we're a completely different team.” In the other parts During the game we didn’t do that and we didn’t look like ourselves.”

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