It was just his first NBA summer league game. Jayson Tatum had miles to go, but there was a lesson he needed to learn right then.

Tatum, the Celts’ top draft pick and No. 3 overall, had posted up and thought he was fouled. When there was no call he said, “Hey, ref, I got hit.”

Derek Richardson, a veteran official, wasn’t having it and had a message for Tatum at the next stoppage, with the ball being inbounded in front of the press table.

“You’re going to be in the league a long time,” Richardson said with a grin. “I’m Derek, you’re Jayson. Let’s learn each other’s names.”

Tatum smiled when the scene was recounted to him.

“I was like, ‘All right, Mr. Derek.’ He said, ‘There we go.’ I just laughed. But he was right,” Tatum said.

That night and since, Tatum has been making the team president, Danny Ainge, look prescient for giving up the No. 1 overall pick and trading down so he could get a future asset and still take the guy he thought was best in the draft.

Tatum, a 6-foot-8 forward, averaged 18.7 points and 9.7 rebounds – summer league games are 40 minutes long – in three Utah appearances, and threw 27 points at the Lakers on Saturday before adding 11 in limited time in Sunday’s easy win against Portland. He’s averaged 9.0 rebounds in Las Vegas.

And though he’s schooled a number of opponents, Tatum is learning all the while. He hasn’t had a lot of trouble but when he has, you can almost read his mind.

He went into the lane against Philadelphia, made his move and got blocked by Alex Poythress. You could see him realizing that particular move won’t work in the NBA.

“Definitely,” Tatum said. “I mean, you’ve got to learn it from high school to college and from college to the NBA. I think that’s what summer league is for, to learn.

“You know, even some shots I do make, I’ve got to think about if LeBron was guarding me or if DeAndre Jordan was down there, they probably wouldn’t go in. I just think about all those things.”

It’s a mature approach, words heard often when Tatum is the subject. Richardson said it later that first night, and the Celtics’ coaches have been impressed. Jerome Allen, who ran the summer-league entry in Salt Lake City, said, “He kind of has an old soul.”

Tatum’s mom, Brandy Cole-Barnes, is proud of her child but isn’t sure she buys every bit of the grown-up talk.

“He’s always been mature for his age,” she said, “but he’s still 19 and I remind him of that sometimes.”

Coach Brad Stevens hopes Tatum continues to display the not-so-rookie leanings he’s shown thus far.

“I don’t know,” said Stephens, who had a rather grown-up rookie in Jaylen Brown last season. “We’ll see. But he does so far seem to be hard to rattle. He moves on to the next play well. You could see that … when he came to work out for us.”

Everyone who’s dealt with Tatum has seen him as more advanced emotionally for his age. His ability to pick things up quickly and avoid mistakes could help get him on the court more often when real games begin.

“I think so,” Stevens said, “but again, we’re just a few days in. It’s so hard to tell. But that’s his reputation.

“You get challenged when you have to play 82 games. You get challenged when you have to ride that roller coaster a little bit. But if you can keep that mindset, it would help greatly.”

Celtics assistant Jamie Young is convinced. “He’s definitely different,” said Young. “His calmness and maturity for his age, his desire to learn, it’s all above average, and certainly for someone who’s still just 19.

“He’s like a sponge. He listens to what you say and takes it in. He’ll ask questions. There are still things he needs to work on and you just know he will. I think he’ll take a step between now and training camp even. And I think he’ll look even better when he’s playing with the guys on our roster.”

Tatum has heard the word ‘mature’ before and shrugs it off.

“I think that’s just who I am, more mature and laid back,” he said. “I think that’s how I’ve always approached the game, never too anxious, always comfortable within my skin. I think that’s helped me out a lot.”

Now it’s about to help out the Celtics.