INDIANAPOLIS — When the normally optimistic Kristaps Porzingis sounds the alarm, that means trouble.

By the time coach Jeff Hornacek tabbed rookie guard Ron Baker to start the fourth quarter at Bankers Life Field House, it was way too late this time. The Knicks were down by 26 points and Hornacek already had his eye on getting back to snowy New York.

After being heroes in Milwaukee on Friday, the Knicks turned into zeros against the Pacers, suffering a letdown of massive proportions in a 123-109 loss. The defeat sent team president Phil Jackson’s club to its seventh defeat in eight games.

Once 14-10, the Knicks (17-20) have fallen hard — as they did last January — and they may not recover because they have become a defensive wreck.

Porzingis said he never felt the Knicks were jelling, even when they were 14-10, instead getting by on talent, and he sounded as uncertain as he has this season in terms of their future prospects.

“In the moment we were four games out of .500, I said it — I don’t see ourselves as that good of a team yet,’’ Porzingis said in his season’s most candid remarks. “We were still growing. We were winning games, but we still had a lot to learn. It was a good moment based on our talent, but we weren’t there yet and now it’s showing. We got to figure this out and keep growing as a team. It’s not coming together yet. It’s frustrating.’’

The Knicks still have three back-to-backs left in January and might want to just forfeit the tail ends. The Knicks are now 0-7 on the second legs of back-to-backs after this sleepy Saturday in the Midwest, when their defense regressed again, allowing the Pacers to shoot 11-of-24 from the 3-point line and 50 percent overall. The Knicks were done in by a 29-14 second-quarter Pacers edge.

Carmelo Anthony, who has not looked good on the second legs, sat out the fourth quarter and finished with 17 points, five assists and two rebounds as the Knicks got hammered on the boards, 53-32. Anthony appeared sluggish early, going 1-for-6 with four assists in the first half before heating up in the third quarter.

Anthony finished 6-of-14 after saying before the game he still is “tolerating’’ playing with a sore right shoulder that sidelined him three weeks ago when the Knicks played at Golden State.

“We were a step slow tonight, especially early,’’ Anthony said. “When I say a step slow, [I mean] to loose balls, rebounds and that effects you offensively. You’re not in sync. We got to figure out how to win on second night of a back to back, especially on the road.”

“To sum up the night, it was defense once again,’’ said Derrick Rose, who sat out the fourth quarter for the second straight game after scoring 14 points with three assists. “They got whatever they wanted. I felt there wasn’t any trust there tonight. It’s all defense. Like I’ve been saying, we have this problem and we are not stopping anyone and we have the mentality we are going to make it hard for them to score.’’

For what it was worth, Baker’s group — in which he shared the backcourt with Brandon Jennings — slashed the 26-point deficit to 11 in seven minutes with their extra effort, but there was not another miracle like in Milwaukee, where the Knicks snapped their six-game losing streak Friday. Jennings notched all 17 of his points in the fourth.

The passion reached a peak with 21.3 seconds left. Jennings and Joe Young had to be separated following a nose-to-nose skirmish after the Knicks guard pushed Young during an inbounds situation. They picked up double technicals and were ejected.

“I was trying to get the ball inbounds, we got tangled up, I pushed him off me and he waved his hand down and looked me up and down,’’ Jennings said. “When he flexed up I was like, ‘What?’ I didn’t take that too kindly.

“I hope he plays next time we play them,’’ Jennings added. “It could be very interesting.’’

Jennings said he barely knows Young.

“That’s why I was, ‘Is this guy serious?’ Like dude, just chill,’’ Jennings said.

Hornacek wasn’t into the drama.

“Sometimes [our passion] comes when we’re down and I’d like to see some of that passion when the game is still going,’’ Hornacek said.

Six Pacers hit double figures, led by Jeff Teague’s 19 points and eight assists. Paul George also added 19. In his second game back from Achilles soreness, Porzingis had five turnovers and was a minus-19 in the first half. Part of the fourth-quarter group, he finished with 16 points and four rebounds.

The Knicks were gruesome in the first half, falling behind 62-43 — Indiana’s largest halftime lead this season. The Knicks also fell behind 29 early in the fourth.

“We got a lot of work to do,’’ Joakim Noah said. “It’s no time to be frustrated. We’re watching a lot of film and it’s a good start.’’