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

Otherwise, getting throttled by the Bulls is a sign of problems even larger than their lack of depth.

After playing two games in a row and coming off a tough defeat against the Thunder, the Knicks started to lose energy in the third quarter of their game against the Bulls, which ended in a disappointing 139-129 loss.

It translated to their first losing streak since early November and arrived because defense turned optional after an extended halftime to honor Derrick Rose.

The Bulls (16-19) dropped 41 points in the third quarter and 76 in the second half. The Knicks (24-12) scored enough behind Karl-Anthony Towns’ 44-point explosion and Jalen Brunson dropping 26 of his 33 in the opening half.

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

Brunson, OG Anunoby and Mikal Bridges — who all played at least 40 minutes the night prior in OKC — were benched for most of the fourth quarter, when the Knicks launched a comeback that failed after the starters returned.

Towns left for the locker room with 1:34 left, appearing to hold his thigh.

There was no immediate information on a potential injury.

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 advantage turned into a 15-point deficit heading into the fourth quarter, and Tom Thibodeau’s squad never recovered.

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

The Knicks starters looked gassed and were dominated, with the Knicks being outscored by 21 points in Bridges’ 33 minutes.

Josh Hart managed just two points in 40 minutes and missed all five of his field-goal attempts.

So it was a celebratory evening for a Bulls franchise that hasn’t had much to celebrate in recent years.

The other reason 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 highlight was an emotional 10-minute speech from Joakim Noah — also a former Knick, albeit a forgettable one — who had Rose in tears.

It was purposefully scheduled to coincide with a game against the Knicks, who not only twice rostered Rose but are coached by his longtime sideline general, Thibodeau.

“It is so well deserved, and I think, for what [Rose] means to the city, the Bulls, the entire NBA,” Thibodeau said. “I had the opportunity to coach against him. So, I know how difficult that is, and then I had the good fortune to coach him. You see in the eyes of the opponent when they had to guard him. You could see the fear and the respect, and then his ability to play his best when his best was needed. In the big games, playoff games, and meaningful games, he went to a different level, and he did it against the best and was very authentic.”

The Knicks played defense Saturday like they were trying to guard a prime Rose. And the entire game felt like a consequence of not enough depth to manage a tiring back-to-back.

Every Knicks starter logged at least 40 minutes in that 117-107 loss to the Thunder — the first time that’s happened for the franchise in the regular season since 2013.

When Thibodeau saw his lead dwindle from eight points to two, he called a timeout three minutes into the fourth quarter, reinserted the starters, and rode them the rest of the game.

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

The Knicks exhausted so many resources building an elite starting lineup, it was left cobbling together a bench from second-round rookies, Precious Achiuwa’s Bird Rights and zero cap space.

The upside is the starters are producing even better than expected, hastening their cohesion by logging so much time together on the court.

But the downside is the Knicks aren’t built to withstand injuries — like the strained hamstring to Miles McBride — or tired legs from the starters.