Early in the fall, after the Celtics’ final roster had been assembled, Celtics coach Brad Stevens knew there would be tough choices to make regarding rotations and playing time. Stevens cautioned that it was inevitable that some veterans would be passed over, and that meant it was inevitable that some veterans would be unhappy.

With the additions of Amir Johnson and David Lee, Boston’s frontcourt was particularly congested, and through the first 28 games of this season, center Tyler Zeller has mostly been the odd man out.

Zeller started 59 games last year and was an important cog as the Celtics surged to an unlikely playoff berth. He is still just 25 years old, so his skills are hardly on the decline. But Jared Sullinger is grabbing rebounds at the highest rate of his career, Johnson has been a force defensively, Kelly Olynyk has stretched the floor with his shooting, and Lee — despite some early struggles — has provided a veteran presence and been a reliable rebounder and passer.

Meanwhile, Zeller has mostly just watched this all unfold from a seat on the bench, as he has played just 150 minutes this season.

“I can’t imagine anybody handling it better,’’ Stevens said of Zeller. “Surely it’s tough. He’s disappointed when he doesn’t play, but he doesn’t show it. He’s been a complete and total professional, and we already know that he’s a really good player. But I can speak for myself and all of his teammates and our whole organization when I’d say we’re all even more endeared to him because of how he’s handled everything.’’

Stevens has also made it clear there would be nights when the Celtics need Zeller, and that they would count on him to be prepared. And Boston’s 113-99 win over the Timberwolves on Monday was one of those nights.

Johnson was sidelined with plantar fasciitis, a painful inflammation of the thick fibrous band on the sole of the foot, and Sullinger sat out because of back spasms. Zeller came off the bench and mostly looked like the effective player he was last season. He made 7 of 10 field-goal attempts and finished with 14 points, 8 rebounds, and a blocked shot. There were signs of rust, too, as he fouled out in just 17 minutes.

“The first half I got three quick fouls, but thankfully coach stuck with me, let me get a bit of rhythm and I got much more time in the second half,’’ Zeller said. “Hopefully I can continue to get more comfortable and continue to get more playing time. But I guess time will tell with that one.’’

Zeller’s cautious optimism stemmed from his recent history. He played well in a loss to the Pistons last week, going 5 of 7 from the field, tallying 12 points and 3 rebounds in just 10 minutes. And then two nights later against the Hawks he did not play at all. Despite his inconsistent opportunities, he is still shooting a team-best 57.1 percent from the field.

He said he just prepares every day as if he is going to play, but he acknowledged that no matter how much individual work he puts in, real game action cannot be replicated. On one play against the Timberwolves, Zeller went up for a layup, felt a defender coming to challenge him, and missed the open attempt.

“I don’t have people running through on me like that when I’m running one-on-zero,’’ he said. “It’s just something you have to get used to and make the adjustments. You need to try to get better every time you go out there.’’

Prior to the injuries to Sullinger and Johnson, Stevens had said he was considering making changes to the regular rotation to help the Celtics snap out of a mild funk. So Zeller may have been in line for a fresh opportunity regardless.

But it is unclear if Sullinger or Johnson will be available when Boston faces the Hornets on Wednesday night, and if one or both are sidelined, it is likely that Stevens will look to Zeller once more. Amid that uncertainty, the only thing that is certain is that he will be prepared.

“I try to work every day,’’ Zeller said, “and try to be as ready as possible.’’

Adam Himmelsbach can be reached at adam.himmelsbach @globe.com. Follow him on Twitter @adamhimmelsbach.