By Brian Robb, CBS Boston

BOSTON (CBS) — At the start of the regular season, Jared Sullinger was a guy wondering where exactly he would fit in among Boston’s crowded frontcourt.

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The fourth-year forward had been a mainstay in Brad Stevens’ first two years with the team, but the acquisition of veteran talent in Amir Johnson and David Lee pushed the 6-foot-9 forward down the depth chart. Tyler Zeller and Kelly Olynyk both outplayed Sullinger for much of the preseason, making Sullinger the fifth big by default by the time opening night arrived.

It was a tough spot for a player entering a contract year, coming just months after Danny Ainge called out the former Ohio State star for his conditioning issues.

Rather than moping about the situation, Sullinger took advantage of his opportunity when it came his way. Kelly Olynyk’s suspension on opening night opened the door for minutes immediately and Sullinger responded with 12 points and seven rebounds in the blowout win over Philadelphia. That performance, combined with uneven play by the majority of the Celtics frontcourt, has led to regular minutes for Sullinger in the past couple games.

The offense has been steady for the big man (nine points per game on 56.5 percent shooting so far), but the biggest change Stevens has seen is on the other end of the floor.

“I told him the other day, I think it’s the best defensively he’s played since I’ve been here,” Stevens said of Sullinger. “That’s been really good. I think the key is to continue to do it and maintain consistency in that. But he’s had some good moments… I think he’s really active. I think he’s very good in the pick-and-roll so far, he’s obviously a very good defensive rebounder. But he’s made a lot of good plays for our team on defense. Sometimes they go on the stat sheet, sometimes they don’t.”

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Sullinger equates the changes to the hard work he put in over the summer with John Lucas.

“I did a lot of things: running, conditioning. I’ve been doing defensive drills, offensive drills… I’m just quicker to react. I’m quicker to react and it makes my job a lot easier when you know you’re in the right spot at the right time — and doing the correct thing at the right time. So it’s very helpful. There was a lot of work I did, and it’s starting to pay off,” Sullinger said.

The 23-year-old also heard the whispers throughout the summer about his disappointing season in 2014-15. He was familiar with the trade rumors and the questions about his work ethic. That talk has given the big man a renewed sense of purpose this season as he plays for a new contract.

“Between a lot of people doubting me and my family semi-doubting me, you kind of have a chip on your shoulder,” Sullinger admitted. “And you just want to prove everybody wrong. I’ve been here going on my fourth year, and things haven’t changed. People still say the same things about you. It’s been like that since high school. It’s been like that since college. And even now. It’s just been me. I keep chipping away and keep doing what I do.”

He’ll have to put more than three solid games together to prove everyone wrong, but Sullinger has already managed to dig himself out of an early season hole. Now, it’s up to him to prove he can remain consistent for a coach that’s looking for a solid rotation in the frontcourt.

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Brian Robb covers the Celtics for CBS Boston and contributes to NBA.com, among other media outlets. You can follow him on Twitter @CelticsHub.