It started all so swell for Tom Thibodeau’s Knicks.

A club that was not touted for its scoring exploded for 66 points in the first half. RJ Barrett started his second season with a big bang, making his first eight shots.

But then the season-opening night turned ruinous. Rookie backup point guard Immanuel Quickley was lost for the second half with a hip pointer and all the night’s electricity short-circuited with that injury.

After taking a five-point lead at the intermission, the Knicks went down with a whimper in the final two quarters as the Pacers routed them, 121-107, in a season-opening defensive dud Wednesday at Bankers Life Fieldhouse in Indianapolis.

The Knicks scored 41 points in the second half as they played oddly detached down the stretch in Thibodeau’s Knicks head-coaching debut.

“The second half I didn’t like the way we played at all on either side of the ball,’’ Thibodeau said. “Obviously what it showed tonight is we’re not good enough right now to win on the road, but I believe we will be.’’

The major bright light was Barrett, who finished with 20 of his 26 points in the first half to go with eight rebounds and five assists. All for naught as the Pacers paid more attention to the No. 3 pick in the 2019 draft in the final two periods.

“I think they played harder in the second half,’’ Barrett said. “I think we had a good first half overall and they came out even more aggressive in the second. I think they did something different defensively with all of our actions. They made really good adjustments at halftime. We’ve got to tighten up the defense for sure. That’s something we’ll go into practice [Thursday] to clean some things up. That’s why we’ve got 71 more to go.”

It could be 71 long games if the Knicks don’t get better center play with Mitchell Robinson and Nerlens Noel falling into early foul trouble and never getting on track. Their interior defense was a mess as Indiana’s All-Star center Damontas Sabonis pummeled the Knicks for 32 points.

The Quickley Effect was noticeable. Thibodeau said he “can’t say’’ if Quickley will be ready for Saturday’s Garden opener against the Sixers and the Kentucky rookie was not made available for postgame comment.

“I think we all saw he had great impact when he went into the game in the first half,’’ Thibodeau said. “But we have more than enough.’’

Quickley came off the bench for starter Elfrid Payton and was a plus-5 in 12 minutes before suffering the hip pointer in a collision with Myles Turner.

With Quickley out for the evening, Thibodeau used no point guard for a stretch of the third quarter when the game got away from his club. Thibodeau finally turned to Frank Ntilikina to start the fourth and he was ineffective.

Not only did Robinson and Noel pick up three first-half fouls each, but Julius Randle (17 points, nine assists, eight rebounds) picked up his fourth midway in the third, forcing him to the bench.

“We just got to put a full 48 together — from top to bottom,’’ Randle said. “The second half wasn’t as good.’’

The second-half collapse soiled a solid Knicks debut for Alec Burks, who scored 19 of his 22 points in the first half. Rookie lottery pick Obi Toppin showed flashes but finished 3 of 12 — all three makes coming on 3-pointers.

Barrett made all eight of his first-half shots — including all three of his 3-pointers — to rack up 20 points in the half. He finished the night 11 of 15 but couldn’t get enough going in the final two quarters.

“I think anytime you have a big first half you’re going to get more attention,’’ Thibodeau said. “And when that happens your responsibility is to make plays.’’

Spurred Wednesday by eight blocks from Turner, the Pacers’ have beaten the Knicks in nine of their last 10 meetings and likely are playoff bound for a sixth straight year.

Rome wasn’t built in a day and neither is the Knicks’ defense. Asked what bothered him the most about their defense, Thibodeau said, “Control of the ball, challenging shots, finishing the defense with the challenge and then of course the rebound. And when you’re small you’ve got to hit people and you’ve got to fight for the ball. We were a little slow reacting to the ball.”