Jayson Tatum had 19 points, 10 rebounds, three steals and one monumental blocked shot in the Celtics’ Game 1 victory against Milwaukee. He spent his rookie year gathering testimonials from those impressed with his talent and maturity, the latter quality brought into greater relief by the fact he was a teenager until early last month.

But none of this is stunning or fortuitous. Given his mother is the force of nature known as Brandy Cole Barnes, the shock would have been if Tatum wasn’t the player — and the man — you’ll see before you in Game 2 this evening.

To understand who Tatum is now, one must understand what he saw as he grew up.

Brandy Cole, already an achiever who’d begun taking classes at the University of Missouri-St. Louis between her sophomore and junior years in high school, had wanted to go to MIT to study biomedical engineering, but the cost would have made that difficult. So she had pretty much settled on a volleyball scholarship to Tennessee instead of an academic ride to UMSL when, just after graduating University City High near St. Louis, things changed.

“When I found out I was pregnant on the Fourth of July, I just stayed home,” she said.

But she didn’t stay in the house. Brandy went to school and began the process of earning degrees in political science and communications.

“I had Jayson on spring break,” she said. “I had to go back the next week and take midterms.”

Later, she would add a law degree. All the while, she held down a job, first as a concierge at a mall, then in commission-based sales for Cingular Wireless and, while in law school, as a grant writer for a nonprofit.

With Cole and Jayson’s father, Justin, never living together and eventually going their separate ways — though Justin remained in Jayson’s life — there was the major matter of caring for the young child. It was a team approach, with her mother playing a huge role, but the buck stopped with Brandy, who would often take Jayson to her classes.

Once, in a large lecture hall, a professor objected.

“I was like, ‘I’m a student, and he’s not disrupting the class,’ ” she said. “There were 200 people in the class, and we’re sitting in the back and he’s quiet.”

Tatum would busy himself with homework or Game Boy.

“It couldn’t have been fun for him, but he wasn’t a fidgety child,” Brandy said. “He even got to play on my team in the law school softball league. He was 9 or 10, and there were people pitching with a beer in one hand. He was like the best player on our team. . . . In order for me to go and participate, I’ve got to bring him with me, so why not let him play?”

The examples set — hard work, no excuses — inevitably took root in Jayson. Washington Wizards star Bradley Beal, whose mother was Brandy’s volleyball coach, saw it all.

“Nothing was given to him. Nothing was ever given to his family,” Beal said. “That’s something I respect about his mom. She’s a true warrior. She took care of him. She loves him to death. She’s his No. 1 fan. And when you come up in that type of setting, you have no choice but to have that mindset that this is how it’s done, and, ‘I’ve got to do it for mom. She’s provided so much for me.’ That pushes you and motivates you. She put all her own dreams on hold.”

It’s why Jayson Tatum proudly calls himself a mama’s boy. The maternal half of that equation is proud, too.

“People ask me about him, and I try to think of things, but he was really almost like the perfect child,” Brandy said. “He’s always been very mature for his age. He’s always been well-mannered. He’s always been respectful. And he’s always been a very athletic child — never obsessed with video games or toys. He just always wanted to go to the gym or play football. He’s always been very locked in and very committed to sports.”

Cole Barnes made sure Jayson was committed in the classroom, as well.

“One time I had to call his bluff, because I would always tell him that we don’t do C’s,” she said. “It was B’s and above because I knew what he was capable of. He would always do fine, but one time he tried me, and I kept him home from a tournament. It about killed him. It was fourth or fifth grade. He almost lost it, but ever since then, he knew I meant business. But he was always really good about that, too.”

The lesson clearly echoed onto the basketball court.

“I think people sometimes underestimate how much work goes into being a top player,” Cole Barnes said. “I mean yeah, he has some natural talent for sure. He’s blessed. But there’s a lot of work that goes into this.”

The quest seems never-ending, but the family did experience a pinch-me moment after making the trip to Cleveland for opening night last October.

“There was so much anxiety with that first game, especially after Gordon (Hayward)’s unfortunate injury,” Brandy said. “But before that, there was my son’s first shot in the NBA, and LeBron comes out of nowhere and blocks it. It was just like, he’s on the floor with LeBron James.

“After that, it was like well, we’re here. This is what you worked for your whole life. And one of the things we’ve always talked about is, ‘You didn’t come here just to be an NBA player.’ That was never the goal. He knew a long time ago he was going to go to the league. Just getting to the league is not our goal. He wants to be one of the best of the best. He wants to leave his mark.”

But while Tatum has excelled beyond expectation, the NBA has left a mark on him, as well. There was a January lull where his shooting and overall energy level waned.

“Don’t get me wrong; he has one of the best jobs in the world,” said Brandy. “But it’s not easy. People underestimate everything that you have to put into it, and they’re not robots. They’re human beings. They’re mortal. People are saying, ‘Oh, it’s a rookie wall,’ or they wanted to blame it on him being a new father. But it’s just being human. He’d never played this many games before and he got tired.

“I’m critical of him, and we’ll talk about it after the game. He’d shot an airball, and I was like, ‘What is the problem?’ And he said, ‘Mom, I can’t feel my legs.’ It can get to you, all the games and the travel. A few weeks ago, he called an Uber to the wrong hotel. It was the name of the hotel he’d stayed at in the previous city. It just all starts running together.”

But he’s still running, outpacing the dreams of the vast majority who are drawn to the game. Tatum came face to face with that early on when, as a second-grader he was asked what he wanted to be when he grew up. The teacher met his “NBA player” response by telling him to pick a more realistic profession.

“He came home and he was devastated,” Brandy said. “He was like, ‘I don’t have anything else.’ He’s always said he didn’t have a Plan B. This was it. It was this or die for him.”

There might have come a time when mom stepped in with some perspective if the dream wasn’t reasonable, but they never got within miles of that moment.

“I saw the work he was putting in and what it was creating,” she said. “So I’m not in awe of where he is now, because he’s earned it. It’s like, what’s the next step?”

The awe will be left to others.