Terry Rozier (left) knocked the ball away from the 76ers' Amir Johnson in the Celtics’ preason win on Monday. Winslow Townson/Associated Press

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Celtics guard Terry Rozier is entering his third pro season, and he said it still feels a bit odd when he looks around the locker room and realizes he’s been on the team longer than everyone except Marcus Smart.

For the past two years, Rozier had leaned on savvy veterans to guide him. They helped teach him how to train, how to play, how to keep his body from breaking down over the course of a grueling 82-game season.

Rozier has sparkled during this preseason and drawn raves from coaches and teammates, and he appears primed to have a substantial role guiding Boston’s second unit alongside Smart.

After the Celtics throttled the 76ers on Monday, coach Brad Stevens specifically praised Rozier’s evolution as a defender. And even though most of the players Rozier leaned on to get to this point on defense are gone now, he said their impact endures.

“I’d never really worked on defense before I got here,” he said. “I never really played defense in high school, and at this level, if you don’t, you get exposed. So I learned a lot of stuff from Avery [Bradley]. I learned a lot of stuff from Jae Crowder. And we lost both of them. So I know I have to step up on the defensive end now.”

Bradley was a first-team all-NBA defender in 2014-15, and he could have earned the honor again last season if he did not miss 27 games because of injury. (His omission sparked social media outrage from countless opponents.)

For two years, Bradley was at Rozier’s disposal. He was happy to offer tips and advice, and when Rozier was on the bench and Bradley was in the game, Rozier studied his movements. Bradley taught Rozier small nuances, like how to place a hand on the offensive player’s hip to subtly force him in the preferred direction.

“Little things like that,” Rozier said. “Or even just working as hard as them when they’re coming off of screens when they’re off the ball. It all goes back to effort.”

Crowder, who sometimes trained with Rozier in Miami during offseasons, offered instruction that was based less on technique and more on takeover.

“Just straight toughness,” Rozier said. “Defense is toughness, and that’s one thing he taught me and provided every game. That’s something that will always stick with me.”

Of course, another way to learn how to play defense is by guarding elite offensive talents. And Rozier also has taken that route in grueling practices, first by checking All-Star Isaiah Thomas over the last two years, and now by attempting to stay in front of another All-Star, Kyrie Irving.

“I love the challenge,” Rozier said. “That’s two of the toughest guards I ever had to guard in my life, and it’s only going to make me better. I love the challenge every day, to wake up and guard guys like that.”

Stevens said it is clear Rozier has become more comfortable with his knowledge of opposing offenses. It takes time.

“You get a little more used to the actions people are running, to where your body should be at the start of actions, to being aware and alert to where your shortcuts are that you can take, to start out with great body position, and that’s just a process,” Stevens said. “Sometimes that gets sped up because you get thrown to the fire a little bit, but it’s easy to take shortcuts. He’s done a good job of just continually learning and growing.”

Rozier’s skills as an offensive playmaker, meanwhile, have been evident since his arrival. But he said he is playing with a different confidence this year, and although it is just the preseason, his jump shot appears to have improved.

In 61 total minutes this preseason, Rozier has tallied 35 points, 14 rebounds, 10 assists, 5 steals and just two turnovers. He has made 50 percent of his field-goal attempts.

“Terry’s probably one of the best guards in the league if you ask me,” Celtics forward Jaylen Brown said. “Just ability-wise, he’s shifty, hard to guard, making the right passes and plays. He’s athletic, defensively getting after it. So, I mean, the more and more he gets time, the more and more he gets reps, the uglier and uglier it’s going to get for people.”