Flagg came away unimpressed with his performance, having missed 16 of 21 shots overall and going scoreless in the second half. The 6-foot-8-inch 18-year-old did add 6 rebounds, 4 assists, 3 steals, and 1 block, which led to a fast break and the go-ahead 3-pointer by Ryan Nembhard in the final minute.

“It was one of the worst games of my life,” Flagg said. “But we got the win and that’s what really matters to me.”

Summer League results are unimportant, and so are shooting percentages and gaudy stats. What every rookie, including Flagg, needs to prove is that he belongs on an NBA floor, that he can contribute consistently. Flagg made those points. Even though many of his shots didn’t fall, he was able to get most of the shots he wanted, using his size and savvy to score over opponents.

He had his moments, especially against James, playing in his second Summer League and looking far more comfortable than in his first, although he finished with just 8 points on 2-for-8 shooting. The NBA pitted the game as Flagg vs. Bronny, and the duo obliged with a couple of one-on-one sequences.

In the second quarter, the 6-2 James blitzed Flagg, stripped the ball away. He then blitzed again, only to be called for a questionable foul. James offered the official a befuddled look, understandably. He wanted the defensive stop. It was a pivotal moment for James, who is still trying to prove he belongs in the NBA and not only because of his legendary father, LeBron James.

Eventually Flagg proved to be one of the best players on the floor, outshining James because of his versatility. While his jumper didn’t fall, and the spin move that worked so effortlessly at Duke will soon be scouted and smothered by NBA defenders, Flagg showed he could play as many as four positions for Jason Kidd’s club.

One enthusiastic observer was former Mavericks primary owner Mark Cuban, who was nothing but smiles after watching Flagg perform. It’s been a difficult time since the Mavericks reached the 2024 Finals before losing to the Celtics. The Mavericks didn’t begin last season with nearly the same momentum, then general manager Nico Harrison made the controversial and foolish decision to trade transcendent star Luka Doncic to the Lakers for Anthony Davis, Max Christie, and a first-round pick.

Harrison has been dragged through league circles for the deal, and the Mavericks missed the playoffs, but that plummet allowed them to enter the draft lottery, where the unexpected prize became the No. 1 overall pick, Flagg.

Cuban couldn’t hide his excitement over Flagg’s potential, texting Kidd throughout the game about Flagg’s ability to play point guard, something he didn’t do often at Duke.

“If he’s able to bring the ball up like he did [Thursday], we’re going to have an enormous lineup,” Cuban said. “He can handle the ball. He knows how to make the right reads. He knows how to make the pass and he can defend smaller guys. He did a good job [running the offense], even when he was tired. So I was excited to see him out there.”

Flagg had 60 ticket requests for the game, and the Mavericks wanted to get an extensive look, considering he’ll likely play only one more game here. He played nearly 32 of the 40 minutes and displayed the type of grit and toughness that may be unexpected from a small-town kid.

The scouting report on Flagg that makes him an irresistible prospect is his relentlessness and propensity for making winning plays, such as his late blocked shot on Thursday. He did not need to score 40 points. His entire arsenal showed he’s ready for this level. But he knows he can be better.

“I really couldn’t get into a rhythm,” Flagg said. “I was a little nervous, a little excited, but just happy to be here. It’s a dream come true. I’m just trying to enjoy the moment. I’m not necessarily worried, but I’m actually just not happy with how it went. But we did get the win.”

There will be bigger openings for Flagg, including when the regular season begins in October, but it’s apparent why he’s considered the best American prospect since LeBron James, and he stepped on the floor still five months from his 19th birthday. He’s a baby with a grown man’s game that will only ascend.

Gary Washburn is a Globe columnist. He can be reached at [email protected]. Follow him @GwashburnGlobe.