SALT LAKE CITY -- "On yo head!"

That's the phrase Boston Celtics assistant coach Walter McCarty bellows in practice when one of his players gets a step on their defender. It's more of a warning, an advisory that the defender might literally have someone on top of their head, as in getting dunked on in an unflattering way.

It's become a more common call during recent practices after the Celtics drafted forward Jaylen Brown with the No. 3 pick last month. Oozing with raw athleticism, Brown has been an explosive and aggressive presence on the floor this summer. He showcased that during his NBA debut on Monday night against the Philadelphia 76ers as he not only got to the free throw line for a whopping 17 attempts, he twice tried to put a 76ers defender on a poster with vicious drives to the basket.

The Celtics already have come to admire Jaylen Brown's explosiveness, which helped him earn 17 free throw attempts in Monday's summer league game. "Jaylen gets a step and he can just take off," Celtics assistant Micah Shrewsberry said. Melissa Majchrzak/NBAE via Getty Images

In the final minutes of Monday's summer league game, Brown got his defender off his feet with a pump fake at the 3-point arc, then stormed into the paint where Philadelphia's shot-deterring big man Richaun Holmes leaped to block his path. Brown nearly threw down a tomahawk jam, but he settled for getting to the free throw line.

Soon after, Brown took a dribble handoff from teammate Terry Rozier and turned the corner into the lane with a head of steam. As the Boston bench rose in unison, Holmes managed to grab Brown's left arm and commit another dunk-saving foul. This time, Brown stared down Holmes after the whistle.

"Next time they might get out the way," Brown suggested after finishing with 16 points, six rebounds and two blocked shots over 28 minutes in Boston's win.

Brown did not play in the Celtics' win over the Utah Jazz on Tuesday night due to a hyperextended knee suffered on Monday. Celtics assistant Micah Shrewsberry, tasked with guiding the summer squad in Utah, stressed that the injury is not considered serious and -- with a day off Wednesday -- the team expects Brown on the floor for Thursday's finale in Utah against the San Antonio Spurs.

The knock on Brown has been that he needs to improve his shot, and that was evident Monday when he missed five of the seven field goals he attempted, along with six free throws. NBA spacing will help him find ways to the basket, but he's got to improve as a finisher.

The more encouraging sign: Brown plays with a mean streak, an aggression not typically present in young players, particularly not at 19 years old.

The 6-foot-7, 225-pound Brown (whose flattop appears to give him a couple of extra inches) may not have been joking at his introductory news conference last month when he declared that he was "ready to rip somebody's head off." Where exactly does that aggression come from?

"I guess it kind of started growing up," said Brown, a native of Marietta, Georgia. "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."

Brown also credits his older brother, Quenton, who played college football at the University of North Texas.

"He's actually trying out for the Raiders now," Brown said of his 24-year-old brother. "He’s much older than me, so he used to beat me up pretty bad growing up."

Now Brown is taking out his frustrations on the young players that compose summer league rosters. Celtics president of basketball operations Danny Ainge has stressed how Brown has an NBA-ready body that defies his young age. Some questioned whether, after one underwhelming year at California, Brown was worth selecting with the third overall pick.

Brown is using that as motivation, while Ainge has pledged great confidence in his rookie.

"We had our eye on Jaylen for a while, a couple years," Ainge said. "So, I don’t know when the moment was, but I will tell you this: The first time I ever saw him play, it’s hard for me to say ‘Wow’ when I watch basketball players because I’ve been around so many great ones in my life. But, yeah, he had me say ‘Wow’ a few times. I have a tendency to look at what they can do and not focus on the things they don’t do great, especially at that age."

As his NBA debut proved, Brown is not averse to contact, though he admits he'd sometimes prefer a wide-open lane.

"I don’t know if I like getting hit, but I try to draw contact," Brown said. "I guess that kind of doesn’t make sense, but, yeah, I’m gonna try to get to the line as much as possible. I’ve just got to finish, hit my free throws, and stay consistent that way. I’m gonna keep getting to the line."

And he's going to keep trying to dunk on his poor teammates, though they'd prefer he save that for game situations when the "On yo head!" shouts are more encouraged.

Alas, Shrewsberry says he keeps hearing McCarty scream it.

"I heard it a couple of times on the bench. Jaylen gets a step and he can just take off," Shrewsberry said. "He’s done it a few times in practice; he’s made those plays really aggressively."