Knicks rookie RJ Barrett completed summer league last weekend with a Las Vegas bang, posting 21 points, 10 assists and eight rebounds. After looking horrendous in the first two games, the former Duke stud got better with each one that followed.

But as his 33.8 overall shooting percentage in the five games showed, there were warts.

Nick Hauselman started a YouTube scouting channel eight years ago called “BBallBreakdown,” in which he uses analytics and dissects technique to determine future NBA potential of college players.

The Post asked Hauselman to get down and dirty on Barrett’s Vegas showing, and he reviewed film on all 125 of his possessions.

There was good and bad, and Hauselman can’t be confident the Knicks have an All-Star in their midst, forecasting a potentially average rookie year.

“I think he can be a solid contributor for a long time,’’ Hauselman said. “I don’t know if we’ll see it this year. He’s not the savior that will add 10 wins.’’

Barrett shot 24% from beyond the 3-point line, but Hauselman thinks that will increase as a rookie.

“His 3-point shot is OK,’’ Hauselman said. “When he hops, he looked definitely smoother than when he does the 1-2 step.’’

Hauselman explained the two ways of shooting 3-pointers — the “1-2 step’’ occurs when catching a pass, putting one foot down and the other forward. Hauselman said the percentages were similar. But …

“On the hop, he seemed in rhythm,’’ Hauselman said. “It’s not broken, going frame by frame. He’s shooting it naturally. When he does the 1-2 step, it was disrupted too easily. That’s repetition, and his trainer, Drew Hanlen, will have a handle on it. He’ll be more comfortable in training camp because he’s a hard-working guy.’’

The NBA 3-point average is in the 36% range.

“I bet he will shoot between 32-33% as a rookie,’’ Hauselman said. “It isn’t horrible for a rookie out of college.

“I’m encouraged,’’ Hauselman added of Barrett’s potential as a 3-point shooter. “I’m more concerned with him getting to the rim. His footwork, his polish. That’s going to be more concerning in my mind.’’

Indeed, that was the sentiment of several NBA scouts on hand who have wondered about Barrett’s transition from being Duke’s lefty slashing sniper to dealing with the NBA’s quicker defenses. It’s more of a natural gift — not as easy to work on as, say, 3-point shooting.

“I don’t think he was blowing by anybody,’’ Hauselman said. “His footwork wasn’t there. I feel like his drives are a little slow and mechanical. James Harden is not an elite athlete and look what he does. But Harden’s skill level is off the charts, and RJ’s still a step below that of Harden’s athletic ability.

“He struggled to get by guys and wound up with a fall-away shot. Maybe he got it to fall. Maybe it didn’t. As a rookie, he’s not going to get calls, either.’’

Hauselman foresees some ugly moments for Barrett on the drive — and that was his bread and butter at Duke.

“I sense we’re going to see drives to the basket where he’ll be stuck in the air, throw it up to get it there and it won’t be close,’’ Hauselman said. “He’ll be looking at the ref saying it was a foul and he’ll have to hustle back on defense. From my mind’s eye, I don’t see a certain economy of efficiency and ability to maximize what you do have athletic-wise. I don’t think he’s there yet. He needs a lot more polish.”

On the positive side, Hauselman said Barrett doesn’t get the credit as a playmaker, especially off the pick-and-roll. Hauselman said his Twitter timeline was filled with patrons complaining Barrett wasn’t passing enough, but the film said something different. The final three games, Barrett became a ball mover. He finished with 21 assists to 14 turnovers.

“I saw good passes,’’ Hauselman said. “He ran some pick-and-rolls, got [Mitchell] Robinson on a roll, threading the needle. Not like he’s going to be throwing crazy passes 50 feet into the corner for a 3, but I saw he made the appropriate pass, saw evidence he’s willing to pass.”

Coach David Fizdale’s offense could be suited for an attacking shooting guard like Barrett — much like it worked well for Tim Hardaway Jr.

“He’ll have opportunities at the shooting-guard position to attack in various ways,’’ he said. “RJ can do the pick-and-roll. He doesn’t have to be just the guy catching and shooting, waiting for the pass.’’

Back to the bad. Barrett rebounded well, but Hauselman didn’t see much on D.

“Nothing stood out as far being a great stopper on the ball,’’ Hauselman said. “Duke has gone to a lot of 2-3 zone. That’s going to be an issue — lack of experience on the ball, man to man, especially at that position. He’ll be playing guys trying to kill him or try to.”