SAN ANTONIO — When the Knicks were at their best in the second half of last season, Nerlens Noel was the starting center and Mitchell Robinson was rehabbing a broken foot.

After 23 games this season, Tom Thibodeau returned Noel to the starting lineup and demoted Robinson, who has struggled with conditioning after gaining muscle weight during his long foot rehab. Noel, who had been spotted in a starter’s blue jersey during Tuesday’s morning shootaround at AT&T Center, made his first start of the season against the Spurs after making 41 starts last season.

Thibodeau wouldn’t confirm pregame he had landed on Noel as the second big change following the wiping out of “The Kemba Walker Era” to start Alec Burks.

But Robinson sounded like he’s headed to a new role off the pine as the Knicks (11-12) entered play on a three-game losing streak. Robinson is seeking a contract extension as he becomes a unrestricted free agent this summer.

“We’ll find that out,” Robinson said of whether he’ll look good on the speedy second unit. “We’ll see how it goes.”

Thibodeau declined to give Spurs coach Gregg Popovich any heads-up.

“Still mulling,” he said beforehand. “Just thinking about everything.”

Thibodeau has across the season talked about Robinson’s conditioning issues as the 7-footer doesn’t look as spry as in his first three seasons, perhaps because of the muscle gain. Robinson said he’s starting to drop weight — from 280 to what he is now at 265.

“I wish I could jump right back into it and be who I was before the injury,” Robinson said Tuesday. “It’s cool. I’m going to work and be back to myself.”

Robinson said “stamina” is an issue. “I’m getting tired real quick,” he added. “After six, seven minutes, it’s boom, I’m gassed.”

Noel has had myriad hamstring/knee issues that have cost him a lot of the season and hasn’t been a reliable defensive specialist. Noel was a team-worst minus-24 versus Denver on Saturday.

Robinson has yet to develop any sort of jump shot and his alley-oop connection from past years also hasn’t been a constant.

“I’m actually getting back to it,” Robinson said. “I’m still strong and I’m still want to get back to myself. I had a lot of extra weight that wasn’t muscle.”

Denver’s Nikola Jokic torched the Knicks in the Nuggets’ blowout win Saturday, scoring 31 points in 28 minutes.

Often, Robinson wasn’t getting out to the 3-point line.

“Yeah, just be in better shape,” Robinson said of what he didn’t do well.

Thibodeau said Robinson’s lengthy absence took its toll. He was lost last March.

“The conditioning aspect is something that he’s still working on,” Thibodeau said. “There’s been some real good moments and moments that haven’t been as good as we would like, but that could be said for our whole team. There’s been some great games and been some games where we didn’t play well. We have to develop a consistency and we’ve got to concentrate on improvement.

“The fact of the matter is he was out so long. He missed all the summer, missed all the fall basically, got cleared right before we started training camp and even then he couldn’t do the entire practice, so it’s been a work in process.”