One of the Celtics’ story lines last summer was whether Jared Sullinger had, in fact, trimmed down his physique and improved his conditioning. The forward posted photos and videos of himself training in Houston with former NBA coach John Lucas, and when he arrived in Boston in the fall he looked about the same as he did when he left the previous spring.

But in the NBA, there are advantages to having a frame like Sullinger’s. Just ask the Celtics’ All-Star point guard, Isaiah Thomas.

“He gets me open, so I love Sully, man,’’ Thomas said. “He gets me more open more than anybody on this team.’’

And it is true that Sullinger has become an invaluable screener for Thomas, helping him get that small slice of daylight that is usually all he needs. But it is also true that Sullinger has been effective in his own right, particularly recently.

Over the last six games, Sullinger is averaging 14 points, 10.3 rebounds, and 3.8 assists while shooting 51.4 percent from the field. All of those marks are above his career highs.

“It’s just playing my game, honestly,’’ Sullinger said. “There was a stretch when I really wasn’t playing well, and now I’m in a stretch where I know where I’m going to get my shots and know when I can create my shots. Just knowing when to pass it up, too. My teammates have a lot of trust in me and that’s the beautiful part about playing for the Boston Celtics.’’

Sullinger has been especially encouraged by his play late in games. There have been times in his career when poor conditioning made him less effective late in games. He has even acknowledged that he sometimes would not give his maximum effort early on, fearing that it would slow him later.

Now, he said, that is not an issue. In the second half of the Celtics’ win over the Heat on Saturday, for example, Sullinger tallied 4 points, 8 rebounds, 3 assists, 2 steals, and a block.

“When you can do what I was able to do the last couple weeks in the fourth quarter, it shows the improvement that I’ve made going along into my NBA pro career,’’ Sullinger said. “I’m looking to improve every day. And just having fun. Having fun winning.’’

Sullinger took over during a key stretch in Saturday’s fourth quarter. First, he received an inbounds pass from Marcus Smart and swiftly fired the ball back to him for a layup. Then he snared a Heat alley-oop attempt out of the air. And finally, after Jae Crowder missed a 3-pointer, Sullinger ripped the rebound away from Miami’s Luol Deng.

For the Celtics, that extra possession ended with Smart being fouled on a 3-point attempt. So during that sequence that lasted just over a minute, Sullinger was largely responsible for helping the Celtics stretch a tenuous 4-point lead to 9. He finished with 12 points and 12 rebounds.

“In the last five minutes of the game,’’ Celtics coach Brad Stevens said, “I felt like he got every rebound that there was.’’

“He helps a lot,’’ Crowder said of Sullinger’s rebounding. “I think that’s what he brings to this team, and he knows we need him to stay on the boards. Amir [Johnson] does a great job as well. Those guys battle the bigger guys for the majority of the game, and Sully gets the best of them almost every time.’’

The Celtics did not extend Sullinger’s contract before last fall’s deadline, so he is expected to become a restricted free agent at season’s end when the Celtics make a qualifying offer. But last week, Sullinger made it clear that he does not want his stay here to be temporary.

He said winning ultimately solves everything. So for now, that will be his focus. He will keep pulling down rebounds and keep pushing through fourth quarters and keep doing all he can to clear space for Thomas.

“Sometimes they are very illegal screens,’’ Sullinger said, smiling. “But it gets him open. That’s all that matters.’’

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Celtics draft pick Marcus Thornton has returned from Australia’s National Basketball League and joined the Maine Red Claws, according to his father, Wayne Thornton.

The Celtics selected Thornton 45th overall last June with the intention of having him play overseas. He signed with the Sydney Kings of the NBL and had some success despite tumult within the franchise. The Kings had a midseason coaching change and their star forward, former NBA player Josh Childress, was hampered by injuries.

“I think it was a good experience,’’ Wayne Thornton said. “The team had some of their own issues, but overall it was a good experience for him.’’

Thornton averaged 12.8 points, 2.5 rebounds, and 2 assists per game, but shot just 37.7 percent from the field and 28.1 percent on 3-pointers.

“They’re a lot more physical in terms of the way they let them play,’’ said Wayne Thornton, who visited his son for three weeks during the season. “It’s a whole different kind of game.

“I think he got a bit stronger. It’s hard to tell whether he made improvements in specific aspects of his game. He worked on his defense, and his shooting wasn’t quite as good as it was at William & Mary.’’

Thornton chose to sign with Sydney in part because the short-season league would allow him to come back to the United States in time to play in the D-League. He spent a week with his family in Maryland upon his return — his first dinner request was Ledo Pizza — and he arrived in Portland, Maine, on Saturday.

“Marcus will have to make some more adjustments to getting back to the way his game usually is,’’ Wayne Thornton said, “and he’ll be all right.’’

Celtics second-year wing James Young is currently with the Red Claws, and the team recently brought back guard Tim Frazier, who was waived by the Portland Trail Blazers.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com.