From across the court, Celtics rookie Jaylen Brown noticed Young’s frustration and fury. He jogged over, patted him on the side, and quietly tried to calm him.

A minute later, at the other end of the floor, Young was whistled for a foul on James Nunnally’s 3-point attempt. At this point, Young was steaming.

SALT LAKE CITY — In the third quarter of the Celtics’ summer league win against the 76ers on Monday, third-year forward James Young drove through the lane and had his shot swatted out of bounds as he fell down. As the crowd gasped, Young was furious that no foul had been called.


“I told him to keep his head,” Brown said. “You don’t ever want to show that you’re frustrated, so I just told James it’ll be all right, just keep being aggressive and that his next game will probably be better. Just encouraging him. I didn’t want him to take himself out of the game.”

It was a simple moment, but it was also a revealing one. Here was Brown, 19, the No. 3 overall pick in this year’s draft, wearing an NBA uniform for the first time in his life, yet displaying the calm and awareness of a veteran as he soothed a player who is entering his third professional season.

“At Cal and in the Pac-12, the worst officiating is arguably in the Pac-12, and it’s safe to say that now, because I’m gone,” Brown said. “There were a lot of frustrating nights I had, and I had to really teach myself, and some of the other guys had to pull me aside and help me adjust and keep my frustration levels down. So it’s just a process.”

Brown sat out the Celtics’ game against the Jazz on Tuesday because of a hyperextended knee, but the injury is not considered serious and he is expected to play against the Spurs on Thursday night.


His maturity and athleticism, mixed with a competitive mean streak, make him one of the more intriguing players in this rookie class, and all three traits were on display in Boston’s win over the 76ers.

Brown attempted 17 free throws — the next closest player on either team took just nine — and continually started fast breaks on his own after pulling down defensive rebounds and sprinting upcourt. In the fourth quarter, he attempted a pair of highlight-reel slams. He was fouled hard on both plays, but the message was sent that he would not be intimidated.

Brown, who is from Marietta, Ga., said he developed his toughness from playing sports against his older brother Quenton, a former college football player.

“I guess it kind of started growing up,” he said. “In the city I grew up, it’s kind of like we all have a chip on our shoulder. Everybody’s trying to get somewhere; everybody’s trying to do something, so that’s just how my city raised me.”

When Celtics assistant coach Walter McCarty sees a player get a step ahead of his defender on a drive to the basket, he has a saying he blurts out from the bench: “On your head!” McCarty is basically saying that the defender will soon be dunked on.

At the Celtics’ summer league practices, Brown has been the one to elicit those calls most often.


“Jaylen gets a step and he can just take off and rise up on people,” assistant coach Micah Shrewsberry said. “He’s done it a few times in practice that he’s made those plays really aggressively.”

Against Philadelphia, two of those fourth-quarter plays simply resulted in hard fouls rather than emphatic dunks. But Brown is confident that it will not be that way for long. He has already seen how floor spacing in the NBA creates lanes and paths that rarely existed during his lone season at Cal. He has already seen what is possible.

“It’s all about execution and mental approach,” Brown said. “I have the capabilities; it’s just all about relaxing, taking my time, and just seeing it.”

. . .

Celtics coach Brad Stevens said the MRI on second-year forward Jordan Mickey’s left shoulder did not reveal a subluxation. Stevens said the hope is that Mickey, who completed a workout in Boston on Tuesday, will be able to rejoin the Celtics for the Las Vegas summer league, which begins Saturday.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.