INDIANAPOLIS — Karl-Anthony Towns sat with his feet in the ice bucket, staring into nothing with a look of disappointment all over his face.

He was locked into this pose for several minutes before verbally expressing his dour mood.

“This is my 10th year in the NBA,” Towns said. “Last year was a year where I had a tremendous amount of experience garnered going to the Western Conference Finals, and I know when we were in Game 80, 81, 82 against the Suns, we looked back at those Game 1, Game 4, you looked back at the ones you gave away that put you in a better position whether it’s to be in the No. 1 seed, No. 2 seed, No. 3 seed, or gives us a chance to choose our destiny. That’s why I’m disappointed.

“Experience has taught me a lot. This is just as important as Game 82. And we just didn’t come out with a win and that’s what’s so disappointing to me.”

It’s still exceedingly early in the campaign, but Sunday’s 132-121 loss to the Pacers was the first time coach Tom Thibodeau and Towns didn’t mostly brush it aside as part of a learning curve. For the third time in their past four games, the Knicks (4-5) were pummeled in the fourth quarter. They gave up way too many 3-pointers, allowing the Pacers to knock down 21 from deep while the Knicks hit just seven.

They lost the math equation.

“They’re a great offensive team. And we gave up way too much,” Thibodeau said. “We don’t challenge shots, they’re going to make. So too big of a discrepancy from the 3-point line. They made 21, we made seven. That’s a problem. Our offense, we score 121 points, we shoot 55 percent. We got to the line a lot. Outrebounded them. But we didn’t cover the 3-point line. So we got to fix it, and we got to fix it fast.”

Defense was always going to be a question mark after swapping out Isaiah Hartenstein at center for Towns. But the Knicks are top-heavy with one of the NBA’s best starting lineups, therefore supposedly better down the stretch.

That hasn’t been the case.

On Sunday, they took a two-point lead into the fourth quarter and were subsequently outscored, 40-27. It was a similar story in Houston and Atlanta.

“If we’re just going to rely on offense, we’re not going to be a good team,” Thibodeau said. “So we have to do better. The fourth quarter is different, we got to treat it differently. We understand how hard we got to play in the fourth quarter. And we have to do better.”

Indiana’s Tyrese Haliburton and Bennedict Mathurin combined for 73 total points. Haliburton, who was a big part of eliminating the Knicks in last season’s playoffs, had his best game of the season with 35 points and 14 assists. Indiana shot 63 percent in the fourth quarter.

“I would say regardless of what’s happening on the offensive end, we can control what we can control — and that’s the defensive side,” Jalen Brunson said. “Offense is not really the problem.”

Offensively, as Brunson said, the Knicks were good enough. Brunson (33 points, 10 assists), Karl-Anthony Towns (30 points) and OG Anunoby (25 points) scored efficiently.

But the porous Knicks allowed the Pacers to make 21 3-pointers — including eight in the fourth quarter — and couldn’t muster one themselves for the first 10 minutes of the final period.

Gainbridge Fieldhouse has been a House of Horrors for the Knicks. Their depleted squad lost all three of their playoff games in the building last season, including the Massacre on Mother’s Day and Andrew Nembhard’s miracle trey in Game 3.

But the infirmary lines were swapped Sunday. The Knicks were relatively healthy, minus the duo of Precious Achiuwa and Cam Payne (both out with hamstring injuries).

The Pacers, meanwhile, were missing three of their top seven — Nembhard, Aaron Nesmith and Obi Toppin.

It didn’t matter against New York’s Swiss cheese defense. Plus, Thibodeau kept his rotation short with Miles McBride as the only reserve logging more than 10 minutes. Then the starters again looked overmatched in the fourth quarter, especially in the 3-point game.

“We have to do better,” Thibodeau said. “And we will.”