CHICAGO – The Knicks better hope it was just tired legs Saturday night.

Otherwise, being throttled by the Bulls and the defense disappearing is a sign of problems even bigger than the lack of depth.

On the second night of a back-to-back after a tough loss to the Thunder, the Knicks ran out of steam in the third quarter in a disheartening 139-129 loss to the Bulls.

It marked their first losing streak since early November and ended with Karl-Anthony Towns limping off the court but likely escaping serious injury after being hit in the leg on his and-1 dunk.

“We didn’t play any defense,” Jalen Brunson said. “Insult was not the problem. Our defense as a group just wasn’t there.”

The Bulls (16-19) dropped 41 points in the third quarter and 76 in the second half.

The Knicks (24-12) scored enough points behind Towns’ 44-point explosion and Brunson, who dropped 26 of his 33 points in the first half.

But the defense disappeared, Chicago shooting 54 percent and getting 33 points each from Zach LaVine and Coby White.

Brunson, OG Anunoby and Mikal Bridges — all of whom had played at least 40 minutes in OKC the night before — sat on the bench for most of the fourth quarter as the Knicks mounted a comeback that fell short after those starters returned.

“Clashes are part of the league,” said Tom Thibodeau, who acknowledged tired legs were likely a factor Saturday. “On top of that, (the Bulls) had to take a break for three days. I knew they would be full of energy. And I thought we scored a lot of goals offensively. Defensively we have to do better.

“If they score and you run back or complain to an official, they speed things up. And this is where we can do better. I knew this would be both a mental and physical challenge.”

The third quarter was the story.

It was a sloppy mess for the Knicks with seven missed treys on seven attempts.

Their nine-point halftime lead turned into a 15-point deficit early in the fourth quarter, and Thibodeau’s team never recovered.

Just like Friday in OKC, Thibodeau maintained an eight-man rotation and his bench was outscored by Chicago’s 31-17.

He doesn’t trust his bank, which was predictable.

The Knicks exhausted so many resources trying to build an elite starting lineup that all they had to do was cobble together a bench of second-round rookies, Precious Achiuwa’s bird rights and zero cap space.

The advantage is that the starters perform even better than expected and accelerate their bonding by spending so much time together on the field.

The downside, however, is that the Knicks aren’t built to withstand injuries — like Miles McBride’s hamstring strain — or tired legs from their starters.

On Saturday, much of that five-man lineup looked gassed and dominated, with the Knicks outscoring 21 points in Bridges’ 33 minutes.

Josh Hart, who scored just two points in 40 minutes and missed all five of his field goal attempts, gave up the tired excuse.

“We scored, but I don’t think the game really got going,” he said. “Tired legs? We’re not even halfway through the season yet, so your legs can’t be too tired. We didn’t execute it. We didn’t play as well as we should have. We don’t deserve to win this game.”

---

---

It was a celebratory evening for the Bulls for several reasons.

The other was Derrick Rose Night, which brought the NBA’s youngest MVP back to the United Center for a series of video tributes, guest appearances from former teammates like Luol Deng and an extended halftime show.

The Knicks came out of that break two steps slow.

“We could make any excuse we wanted if we wanted to,” Towns said. “Ultimately, OKC found a way to beat us. Chicago found a way to beat us. It’s that simple. We could apologize for every little thing. But at the end of the day they found a way to beat us.”