By Matthew Geagan, CBS Boston

BOSTON (CBS) —  Thursday night in Cleveland was all about moral victories for the undermanned Celtics.

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Without Al Horford and Jae Crower, the Celtics had little shot against the defending champs. LeBron James was LeBron James, the best player to bounce a basketball on any planet, and dominated his way to 30 points (20 in the third quarter alone) 12 assists and seven rebounds. Kyrie Irving hit big shot after big shot, and the Cavs did their best Amir Johnson impression from beyond the arc, draining 14 of their 38 attempts from downtown.

Keeping pace with the Cavaliers without two of their key contributors is a tall task for any team. The Celtics still managed to make it somewhat interesting in the end, with the final tally 128-122 in favor of the Cavs.

But the biggest test of the evening went to 20-year-old rookie Jaylen Brown, who in his first career start as a professional basketball player was asked to waltz with James. There were rookie moments from Brown, as even savvy veterans are prone to against a once-in-a-lifetime talent like James. But the rookie held his own, and at times looked like he was one of those savvy vets. If anything, Brown proved that he belonged in the NBA.

Even LeBron took notice, and The King approves.

The rookie said he wasn’t fazed or starstruck with Thursday’s assignment, and it showed. He finished with 19 points off 8-for-16 shooting, including 3-for-6 from downtown. He pulled down five rebounds in his 36 minutes on the court, including a pair of offensive boards. One of them had the interweb buzzing about the 6-foot-7 forward’s athleticism as he out-muscled Kevin Love under the basket:

As for his tussles against James, things started off well as Brown posted up against the reigning Finals MVP, not backing down from a 1-on-1 opportunity. James drove to the paint, and Brown followed step for step. The result was an early James miss. There weren’t many more of those, but it was clear that Brown had not just accepted the challenged, but was embracing it.

Brown didn’t back down on the offensive end, either. Of his eight baskets, five of them were the result of the rookie attacking the rim. That included a second-quarter drive in which he pulled a, well, LeBron James on LeBron James.

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The rookie got the veteran to bite on a three-point fake, and quickly took off for the hoop. He delivered a two-hand jam that Celtics fans won’t soon forget. And neither will James.

When the final buzzer sounded, the veteran tracked down the rookie to share a few words. Those exact words remain a mystery, but James expressed his esteem for the Celtics’ young forward to reporters after the game.

“I’m not going to tell you what I told him. If he wants to bring it up, that’s fine,” James told reporters after the game. “But I think he’s a really good talent and that’s why he was drafted so high. He’s a strong kid. You can see he knows how to play the game.”

“He just said he’ll see me down the road,” Brown said of James’ words of wisdom. “He said ‘good job,’ and he said some other things that I ain’t gonna mention.”

Brown knows he’s still a rookie with a lot to prove in the league. But after just 99 minutes on the floor, it’s clear he has the mindset that he belongs. And with his play, he’s proving to those who disagreed when the Celtics drafted him third overall in June.

“It’s time to play basketball. It’s not a game, it’s not a movie. This is real life,” he said. “You’re here in the NBA, your feet are set, now it’s time to make a name.”

Thursday night was an extremely early test for a kid who just a few months ago was playing against other kids at Cal. Now he’s going up against the men of the NBA, and in Cleveland, he was tasked with keeping up with the man of the league.

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While the Celtics did not earn the victory, Brown passed his first big exam, a solid moral victory on a night of moral victories. There will be plenty of rookie moments throughout this season, but when LeBron James takes notice, you can bet the Celtics have something special in Jaylen Brown.