Celtics rookie JD Davison is on a two-way contract and will spend time with both Boston and the Maine Celtics. John Locher/Associated Press

JD Davison drove to the bucket in one of his first Celtics practices. The rookie initially thought he had a wide-open lane, but that realization only lasted a split second.

This is the NBA – not college basketball. And Davison is a Celtic, so he was matched up against Marcus Smart, the league’s reigning Defensive Player of the Year. That’s when Davison said it sunk in for the first time he’s a pro: when Smart put the clamps on him in practice.

“He sprinted, turned right back in front of me, and I’m like, ‘Holdup, now,’ ” Davison said with a smile. “Welcome to the league.”

A glance around the Auerbach Center practice courts shows the Celtics brimming with talent, experience and confidence. That’s natural for a title contender. There are plenty of established NBA veterans who are improving their game and chemistry. Then there’s Davison, the 20-year-old second-round pick who’s the only rookie at Celtics training camp.

But Davison said there isn’t as much nerves as there is excitement. It’s an educational season for him as he’s on a two-way contract, so he’ll bounce between Boston and the G-League Maine Celtics throughout the season. But Davison will be there in practices, and he’s soaking in every little bit of information he can get. There hasn’t been any rookie hazing at camp yet, though Jayson Tatum did make him buy donuts when the Celtics flew to Las Vegas for Summer League.

The early returns have been promising. Davison played in six minutes in the Celtics’ preseason win over the Hornets on Sunday, putting up four points and four assists. He played under control in his first time on an NBA court. It’ll take a little longer for the rookie to feel comfortable, but that’s all a process.

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“I ain’t going to get starstruck by my teammates,” Davison said. “But I’m a shy kid. I don’t talk too much. Once I got comfortable around here, I’m talking, they’re talking back to me, smiling. So I just treat them guys like my brothers to me.”

Luckily for Davison, he has a willing mentor in Smart. The two play the same position, and Davison said he talks to Smart every day. They sit next to each other in the film room. Smart said he sees himself in his new mentee, which is why he introduced Davison to the league so quickly.

“For JD, I definitely came in a little earlier on him and showed him a little early just to get him ready for what he needs to expect,” Smart said. “Not everybody’s like me. Obviously I’m a different type of person when it comes to that end compared to most other people on that end. If you can deal with me, you can deal with anybody. He’s going to be great. I’m glad to have the opportunity to take him under my wing.”

Davison, who struggled at times in college at Alabama, is still a raw product. But the talent and athleticism are clearly there. He put up a 28-point, 10-assist double-double in Summer League, where he played within himself, looked mature and got buckets.

There’s also plenty of time. The Celtics have Smart, Malcolm Brogdon, Derrick White and Payton Pritchard all on the active roster as guards ahead of Davison. They don’t need Davison to rush his development, instead using this as a “learning” season where he picks everyone’s brains whenever possible.

“I talk to all of them, not just Marcus,” Davison said. “Just seeing those guys compete against each other is what pushes you harder. All of them want to make all of each other better. So they’re going to make it better for our team but for them also. Just seeing those guys go at each other made me want to jump in there and go at them, too.”

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Smart, 28, said he remembers when he was Davison’s age. Now in his ninth year in the league, it’s Smart’s turn to help raise the youth. But Smart didn’t have a veteran he could rely on early in his career, which is why he wants to be there for the young guy in the Celtics locker room.

ROBERT WILLIAMS III’s return is still weeks away from returning after knee surgery, but he’s slowly making progress. Interim coach Joe Mazzulla said after Tuesday’s practice that Williams shot free throws, so there’s at least some positive signs there.

Williams has also been consistently around the team, whether that’s getting treatment or watching film with his teammates. That’s all in preparation for Williams’ return to the court.

“He’s in a great mind frame,” Mazzulla said of Williams. “As guys are getting better on the court, he knows the treatment room is his basketball court right now. So he’s doing a great job of getting treatment and kind of building towards that.”

Williams underwent an arthroscopic procedure right before training camp to address swelling in his left knee. He had been dealing with knee issues in the playoffs after surgery late in the last regular season, but the operation was a surprise.