The Celtics learn more from losses than wins. What can they learn from their latest?

With his jumper struggling, Jayson Tatum (right) started forcing things again against Indiana.Darron Cummings/Associated Press

There are more lessons to be learned from defeats than victories and the Celtics certainly hope their thumping at the hands of the Indiana Pacers provided some knowledge for the rest of the regular season.

The Celtics made plenty of mistakes Sunday at Gainbridge Fieldhouse, playing down to the level of their opponent, sleepwalking early defensively, and allowing Indiana to feel comfortable — the biggest indictment was the reversion to the early-season habit of allowing poor offense to result in poor defense.

Most concerning was Jayson Tatum, who seems intent on allowing his 3-point success to determine the rest of his game. He attempted 12 threes, more than he’s taken in a month, and made just two. Instead of attacking the basket, Tatum settled for jumpers, determined to prove he can knock them down.

What was bizarre, however, is that Tatum appeared hesitant to shoot threes in the fourth quarter. On one possession, Derrick White set Tatum up for a wide open look, and he pump-faked, dribbled, then passed it off. Tatum then passed up another open 3-point look only to force up a step-back to beat the shot clock.

It was the type of disjointed basketball that was prevalent early in the season, forcing matters when they don’t come easy. Things weren’t going to be easy Sunday; the Celtics were playing in the second game of a back-to-back set with another funky start time — 5 p.m. –—against a revamped team they knew very little about.

But it’s the type of game they still need to win, regardless of style points. The Pacers were 20-41 and had beaten just one Eastern Conference opponent since Dec. 16. The question is whether the Celtics’ defensive slippage will continue as they face a daunting stretch of games against the Atlanta Hawks, Memphis Grizzlies, and Brooklyn Nets, the last of which should feature Kyrie Irving and Kevin Durant.

“After a game or two, I wouldn’t say that we’re fallen off a cliff or anything,” Udoka said. “We just have to refocus and kind of settle back in and have the starts that we had earlier in the streak. Not to say the break halted our momentum or focus or anything, because we still have guarded well at times as you saw in Detroit in the fourth quarter and certain quarters, so [we] just have to come out with a better mind-set and play through a three [games] in four nights. It happens to everybody, and you have to go through those during the season.”

Ime Udoka didn't sound worried after Sunday's loss. "We just have to refocus and kind of settle back in and have the starts that we had earlier in the streak," he said.Winslow Townson/Associated Press

The Celtics have won with their increased focus, punching first and not allowing opposing teams to get comfortable offensively. On Sunday, the Pacers — boasting an average offense at best — scored 18 points in the first 4 minutes and 40 seconds. It’s a prime example of what can occur when the Celtics aren’t completely locked in.

Tatum looked frustrated from the tip. Jaylen Brown got upset when Indiana’s Goga Bitadze threw him to the floor during a post up. Marcus Smart spent the postgame trying to reason with officials about some questionable calls.

“The schedule was interesting, unique, can’t make any excuses,” Brown said. “Two early games and a back-to-back on the road. I don’t think I’ve experienced that since I’ve been in the league, still not an excuse.

“We’ll get it back. No reason to sulk. No reason to complain, got some big games coming up, let’s look forward to those.”

The question was whether Sunday is just an aberration or the beginning of a troublesome trend. The Pacers’ 52.2 percent shooting from the field was the first time a Boston opponent had shot above 48 percent since the Charlotte Hornets Jan. 19. What’s more, prior to Indiana, only one opponent during the Celtics’ string of 11 wins in 12 games made more than 45 percent of its attempts (Detroit on Feb. 16).

Boston’s defense had been stellar before the Indiana game, and that defense will be tested over the next few weeks.

“Our mind-set, still keep hanging our hat on defense,” Brown said. “This is the NBA, teams are going to make shots. But we give ourselves a chance in any game because our defense gives us a chance. If we can guard, I think we’ll be all right. Looking forward to being really good on the defensive side for that stretch.”

Jaylen Brown believes good defense remains the priority for this Celtics' team.Matthew J Lee/Globe staff

The Celtics spent the past month getting themselves out of doldrums after their wild inconsistency during the first half of the season. Now it’s time they ramp up their play and make a serious run at home-court advantage in the first round. Sunday was a missed opportunity against an inferior opponent.

March will be a challenging month with the Hornets, Mavericks, Warriors, Nuggets, Jazz, Timberwolves, Raptors, and Heat on the schedule. No more cupcakes or downtrodden teams.

“Everybody’s going to chime in with what they think, that’s fine but that’s out of our control,” Brown said. “For us as a group, we try to keep the outside pressure outside and just come in and play basketball. Just hoop, do your job well and just stay the course. And that’s brought us into good moments in the season, even in the midst of adversity. I look to do nothing other than that now. Just hoop. Breathe. Play good basketball and we’ll be fine.”