One quality that becomes more apparent with each game is the reservoir of confidence Jaylen Brown has brought to his NBA career.

It’s one reason Brad Stevens felt comfortable plugging the rookie into his starting lineup Thursday night in Cleveland as a temporary patch for the injured Jae Crowder.

“Nineteen years old and you’re the third pick in the NBA draft, so you’re probably a pretty good basketball player usually,” the Celtics coach deadpanned. “I think he’s kind of grown up thinking that, and that’s a good thing, and today what was better was how detailed he was on some things, especially defensively.”

Brown has absorbed a lot of information in a short amount of time, certainly. For a coach as detail-driven as Stevens, that may be the only way for a rookie to crack the regular rotation.

But Brown’s basketball IQ — indeed, his smarts in general — is driven by tremendous self-belief. In a short amount of time, going back to the exhibition season, he has guarded Cleveland’s LeBron James, New York’s Carmelo Anthony and Chicago’s Jimmy Butler, all with varying success.

Brown, though, believes he belongs in these matchups.

“There’s no back-down in him,” said teammate Isaiah Thomas.

Brown dunked on James during the Celtics loss Thursday night, and was later complimented by the Cavaliers star, who also took the opportunity to take a jab at Brooklyn for surrendering the No. 3 pick that brought Brown to Boston.

“Boston definitely is enjoying all the picks that they’re getting from the Nets the last few years,” James cracked.

It’s virtually impossible for a rookie not to melt a little bit when paid this kind of compliment by the best player on the planet.

“I guess it means a lot. I respect LeBron not just on the floor but off, what he does off the floor donating to inner city kids, how he handles himself,” said Brown. “Never been in trouble with the law or anything like that. He’s just a great role model for somebody like me and I watched him growing up, and he’s kind of shaped how I am now.”

Brown knew better than to let too much of that respect show through, though.

“It’s different when you get in between the lines, all that goes out the window,” he said. “At the end of the day I’m a competitor, and I think I’m more competitive than a lot of people in this league. I had a focus, I wasn’t nervous, I wasn’t star-struck or anything. I came out and had a job to do and tried to do it to the best of my ability.

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

Stevens looks deeper, of course. Beyond the 19 points and 3-for-6 night of 3-point shooting was an adherence to all of those details. Some rookies will never master these final points. Many are incapable of processing everything at the age of 19.

Brown, though, seems to get it at the earliest stage of his NBA career.

“Brad’s favorite quote I took a liking to is small hinges swing big doors,” said Brown, who is already speaking Stevens’ language. “The attention to detail is very important. I try to pay attention to it as much as possible.

“I talked to Al (Horford) a few weeks back and he told me that if he could do it again when he was 20, 22, he would have paid attention more to those small things,” he said. “That’s what I’m paying attention to right now.”

And he’s already left an impression on his veteran teammates.

“You have to be confident to be in this league. I’ve never seen him have a bad game,” said Thomas. “It’s early, so you don’t know about the confidence thing. But he hasn’t shown no back-down, not even from guys in practice from Day 1. Going against a guy like LeBron, your first start, against him, he didn’t show no back-down as well. Those are all good signs of a special player.

“He’s talented. He’s a guy who can play the 3 and the 4, he’s very strong and very athletic,” Thomas said. “Once he figures out how to play he’s going to be special, because he’s a guy who works. He’s in before and after practice. He’s very smart. He wants to learn. Those are all good signs.”

Until Brown assumes that larger role, though, he’s absorbing the best experiences the NBA can offer — the opportunity to guard and score on superstars.

“You can’t get too much better than LeBron James and Carmelo, Jimmy Butler,” he said. “Those are some of the best players in the league. I know some other players out there pretty good as well, but once you face those guys, how good can you get?”

The Celtics still aren’t sure, but five games into Brown’s rookie season, they probably have a good idea.