During Saturday’s open scrimmage at Columbia University, RJ Barrett made what broadcaster Mark Jackson would call “a grown-man move.’’

The Knicks’ 6-foot-7, rookie shooting guard drew contact with defender Frank Ntilikina after barreling down the lane, but the former Duke great rolled in the layup as the whistle blew for a foul.

Barrett’s “and-1” against a skilled defender was no fluke. While perimeter shooting is a work in progress, Barrett should make his mark inside. even at age 19.

“He’s a strong kid,’’ coach David Fizdale said. “He’s stronger than college kids. His strength is already NBA-ready. I can see him really being a force in the paint.”

Barrett’s outside shot was wobbly at Las Vegas summer league in July (24 percent from 3), and he was just 1-of-4 on jumpers during Saturday’s two 8-minute periods, making one trey. But Barrett still has the sturdiness and smarts to make plays.

Barrett also plays dogged defense, with rebounding coming naturally. Saturday, Barrett intercepted a deep pass after getting back on defense after he bricked a 3-pointer.

“His shot is progressing slowly but surely,’’ Fizdale said. “He had one brick but made a couple of good shots and made plays for teammates. I like how he competed defensively. He shares the game, knows how to play.”

“He doesn’t remind me of any rookies. It seems like he’s already done this before. I feel he’s ahead of the game to most rookies I’ve coached.”

Barrett said his Knick mentors have been Marcus Morris and Wayne Ellington. Asked about his ability to muscle in for buckets, Barrett said, “God blessed me with this body I have. I’ve been working on it all summer. I feel I’m physically ready.”

PG Dennis Smith Jr. did not practice because of a strained lower back. He suffered disc woes late last season, costing him games, but Fizdale thinks he’ll return to practice Sunday and be ready for the preseason opener in Washington on Monday. … Fizdale said he’s looking at Julius Randle to play some point forward. “I’ve done it for a while, bringing the ball up,’’ Randle said. “I did it with Alvin (Gentry in New Orleans).’’