Knicks guard Jalen Brunson drives past Hawks forward De'Andre Hunter...

Knicks guard Jalen Brunson drives past Hawks forward De'Andre Hunter during the first half of an NBA game Friday in Atlanta. Credit: Hakim Wright Sr.

ATLANTA — If there was a mystery regarding what life would be like for the Knicks as they embark on what likely will be at least a month without Mitchell Robinson, Onyeka Okongwu put an exclamation point on the answer.

With 7:19 remaining and the Knicks fading fast Friday night, Julius Randle, shifted over to center, reached for a rebound. But Okongwu reached over him, grabbed the ball and threw down an uncontested reverse dunk.

Tom Thibodeau tried all kinds of combinations, with Jericho Sims starting at center and Isaiah Hartenstein seeing significant time at the position, but the gaping hole in the defense was evident as the Knicks fell to the Hawks, 139-124.

Okongwu’s follow dunk was just one lowlight in a fourth-quarter collapse. The Knicks trailed 102-101 at the start of the period, but their 1-for-10 shooting slump gave the Hawks an opening and they blew through it, going ahead by as many as 19 points in the quarter.

The Hawks shot 56-for-100 from the field and didn’t score fewer than 33 points in any quarter, and their 37-23 advantage in the fourth period broke open the game.

Knicks nemesis Trae Young had 26 points in the first three quarters but didn’t join the fun until he converted a technical free throw with 3:58 remaining.

The Hawks got 29 points from Dejounte Murray and contributions up and down the lineup. The win was the fifth straight for Atlanta after a chaotic start to the season, and paired with the Knicks’ three consecutive losses, the Hawks moved within a half-game of them in the Eastern Conference standings.

Randle finished with 32 points, nine rebounds and six assists for the Knicks, who shot 49-for-83 (59%). RJ Barrett added 23 points and Jalen Brunson had 19 points and nine assists.

This was the first test for the Knicks of what will be an extended absence for Robinson, and while their offense remained fine without him, the missing defense and rebounding was evident from the start.

“You give up 139, and credit to them, they’re playing great basketball right now, but we’re not gonna be able to win a game like that,” Thibodeau said. “And so I liked our offense a lot, shoot 59% and score 124. But right from the start, there was no defensive urgency or mindset from the start of the game.”

“It’s hard to cover for so much,” Randle said of Robinson’s absence. “You kind of forget when he’s not out there how much he’s covering. He covers a lot and obviously we would love to have him, but it’s gonna be a collective effort trying to make up for what he brings.”

The Knicks shot 16-for-20 in the first quarter but managed just a 38-35 lead as Atlanta piled up six offensive rebounds and nine second-chance points without Robinson on the floor.

The Knicks’ hot shooting continued as they ran it to 24-for-30 before cooling off to 71.4% for the half. But Atlanta hung with them, and the Knicks escaped to the locker room with a 71-68 halftime lead.

Barrett led the way with 6-for-6 shooting and Immanuel Quickley was 4-for-4 off the bench. Every Knicks player who entered shot at least 50% in the half, and Randle, Barrett and Brunson totaled 43 points.

But the Hawks ran off a 13-0 spurt in the opening 2:59 of the third quarter, with Young scoring nine of the points. He drew a fourth foul on Quentin Grimes — and a flagrant in the process, as Grimes came down in Young’s landing area on a three-pointer — in what turned out to be a five-point play for Young.

Randle, however, picked up his play and outscored Young 17-14 in the quarter. The 17 gave Randle 32 points entering the fourth quarter, but he wouldn’t score again.

Sims, who most replicates Robinson’s skill set as a shot- blocker and leaper around the rim, got his starting assignment. But Hartenstein certainly will be in the mix, having started seven games at center this season.

“We’ve gone back and forth,” Thibodeau said. “I feel good about both guys. Part of it was to try to keep some rhythm with the second unit. It obviously in some ways didn’t work out because of the foul trouble Quentin had. Sometimes you plan and things happen during the game that change what you’re planning on doing.”