Kristaps Porzingis began what the Celtics called the “next phase of his recovery” on Monday, joining Boston’s G League affiliate for a practice at the Auerbach Center.

Head coach Joe Mazzulla said the rehabbing big man suffered no setbacks during that session.

“I only watched a little of it, but he came out of it OK,” Mazzulla said before Tuesday’s NBA Cup matchup with the Cleveland Cavaliers at TD Garden. “So he’ll just continue to ramp it up and get better and better.”

Porzingis underwent offseason surgery to repair the rare leg injury he suffered during the NBA Finals. He has not played since, but he has been around the team and, by all accounts, is making steady progress.

Mazzulla didn’t explain exactly what this “next phase” for Porzingis entails (“I have no idea. It’s a medical phase”) but said he’s been pleased with the work the 7-foot-2 center is putting in.

“All I know is he’s getting better every day, and he’s working really hard at coming back as fast as he can,” Mazzulla said. “And he’s in the next phase of the ramp-up.”

Porzingis was the only Celtics player unavailable for Tuesday’s game. Sixth man Payton Pritchard was listed as questionable Monday with a sprained left thumb but was upgraded to available before the game.

Tillman Maine man

Joining Porzingis for his brief G League rehab assignment was forward Xavier Tillman. Some players might balk at a request to practice with the farm club, but Mazzulla said Tillman, who has fallen out of Boston’s rotation of late, embraced the opportunity.

“He just wanted to play,” Mazzulla said. “It was an opportunity to play, and he’s obviously done a great job of just doing what we’ve asked him to do. So it’s a credit to him. We’ve got a lot of respect for him for making that decision. He’s got to keep getting better and better. He’s brought great character, a great work ethic to it.”

Acquired from Memphis ahead of last season’s trade deadline, Tillman logged substantial minutes over Boston’s first four games but has hardly seen the floor over the past three weeks. Entering Tuesday, the 25-year-old had played just 19 total minutes over the last nine games – despite starting one of those – with four healthy DNPs during that span.

The bulk of Tillman’s minutes have gone to center Neemias Queta, who’s emerged as a steady frontcourt presence for Boston after splitting last season between the NBA and G League.

Queta hasn’t replicated Porzingis’ rim protection and isn’t a perimeter shooting threat, but he entered Tuesday ranked third among Celtics players in rebounds per game behind Jayson Tatum and Jaylen Brown and first in rebounds and blocks per 36 minutes.

Cavs hurting

Mazzulla said the biggest difference between the current Cavaliers, who carried a perfect 15-0 record into Tuesday’s clash, and the Cleveland squad Boston dispatched in the Eastern Conference semifinals was that this one is “healthier.”

At the top of their roster, that’s certainly true. The Cavs were without Donovan Mitchell for two games of the teams’ playoff series and down Jarrett Allen for all five; both have been available and highly impactful this season.

But Cleveland was missing several important role players in this rematch, with Isaac Okoro, Dean Wade and Caris LeVert all sitting out due to injury. Okoro and Wade have made 15 combined starts this season, and LeVert is the top bench option for head coach Kenny Atkinson.

Atkinson, who didn’t shy away from the hype surrounding this matchup, said playing against the defending champion Celtics would give the Cavs valuable “feedback” about their own championship bona fides.

“Hopefully, we’ll see this team down the road,” he said. “(This is) an important game. On the other hand, I don’t want to blow this game out of proportion. It’s that balance, but it’s more anxious to see where we are. And we all know our schedule has not been the hardest. So we’re playing an elite team, the top team in the league. So I just can’t wait to get some feedback from the game.”

Mazzulla on Montgomery

The Celtics’ fellow TD Garden tenant underwent a coaching change Tuesday, with the reeling Bruins firing Jim Montgomery 20 games into his third season with the club.

Mazzulla, who’s spoken previously about how Boston’s head coaches support each other, called Montgomery’s dismissal an unfortunate reality of their business.

“I talked to him a couple times,” Mazzulla said. “It’s tough. I talked to him a few times, and then watching the games. I didn’t really dive deep into his coaching philosophy or stuff like that. You hate to see a coach have to go through that, but we all get hired to get fired.”

Montgomery and Mazzulla found similar regular-season success in Boston, but each of the former’s first two seasons ended in an early playoff exit. The Bruins then nosedived this season, prompting management to cut bait after an 8-9-3 start.

Joe Sacco will replace Montgomery on an interim basis.

Off the rim

Big Boston sports week for Chris Sale. After attending Patriots-Rams at Gillette Stadium on Sunday, the former Red Sox ace was among the notable attendees at Celtics-Cavs. … Boston’s remaining schedule for NBA Cup group play: at Washington this Friday and at Chicago next Friday. Unless the Celtics make the knockout rounds and earn a home game in the quarterfinals, Tuesday was their final time playing on the bright-green TD Garden court.