BOSTON — While the rest of the Boston Celtics’ roster is healthy heading into training camp, there is one big exception in Kristaps Porzingis.

But that was expected.

The Celtics’ big man had surgery to repair his rare foot injury from the NBA Finals and will be sidelined through training camp and beyond.

By all indications, Porzingis only had optimistic words Tuesday at Celtics Media Day. He shed some light on the rehab process, though he didn’t give a concrete timeline on his return. But the positive tone certainly is a good sign for the Celtics.

“It was a rare injury,” Porzingis said. “Got the surgery done right after the parade. And then it was three, four weeks or so, kind of at home hanging out. And then was able to start doing more in August. Now it’s end of September already. Been doing a good amount of stuff, doing pretty good. Looking forward to keep making the progress at this rate and hopefully be back out there with the guys as soon as possible.”

Porzingis told ESPN he hopes to be back on the floor in December. The Celtics want to make sure he’s 100% when he comes back. Al Horford will be the likely starter with Porzingis sidelined – just like during the playoffs – but the team will keep the long-term picture in mind.

Boston will lean on its trio of reserve big men – Luke Kornet, Xavier Tillman and Neemias Queta – for depth. The team re-signed all three of them over the summer, as it was clear the Celtics wanted to shore up that part of their roster. If there is another big man injury, the Celtics also have their 15th roster spot open, just in case. But thus far, Porzingis’ rehab has turned heads in the front office for positive reasons.

“He’s very optimistic, I think,” said Brad Stevens, president of basketball operations. “I don’t know that we’re interested in putting a timeline on him because the injury is unique. But as far as how he feels and the progress that he’s made, I’d say we’re very, very pleased with where he is and maybe a little surprised.”

Porzingis said he’s been running and walking without pain. He’s also already getting in some court work, which he’s shown glimpses of on social media. And he’s been doing some live contact, which is why Porzingis and the team have been optimistic thus far.

The Celtics know how to play and win without Porzingis, which they showed during their title run. But they aren’t whole without their big man making an impact on both offense and defense.

“Obviously the energy I’m feeling from the doctor, the surgeon, the medical staff is not to rush anything,” Porzingis said. “For myself, obviously, if you ask me and I had to do my own protocol, I would probably go quicker, but that’s a good sign. I’m feeling really good. My ankle’s feeling really, really good, and the quicker or the sooner I can get the green light to do more and more stuff, I’ll be happy and I’m sure I’m gonna be more than 100% ready for each stage that’s coming up now.”

PORZINGIS WASN’T the only Celtics’ big man who went under the knife this offseason. Tillman and Kornet also had surgeries but have fully recovered and are expected to participate in training camp.

“Xavier had a knee scope in the end of July and is doing great,” Stevens said. “He’ll be full-go for practice (on Wednesday) and has been playing for the last week or so. Luke had his wrist operated on right after the season. Same thing – cleared to play.”

Having Kornet and Tillman available early in the season will be important to replace Porzingis. At age 38, Horford’s workload needs to be managed, which will create opportunities for Kornet, Tillman and Queta.

“Those timelines matched up well with the start of the season, obviously with some big depth issues, potentially,” Stevens said. “So we’re fortunate that our training staff and those guys, with the effort they put in to get back, have both done a really good job of building it up so that they’ll be ready to go when we need them to go (on Wednesday).”

Kornet, a 7-foot-2 journeyman, appeared in 63 games last season and set or tied career highs in field-goal percentage, free-throw percentage, rebounds per game and blocks per game. The defensive-minded Tillman came over from Memphis in a deadline trade in February.

JAYSON TATUM admitted his limited playing time at the Olympics was difficult for him to handle. But he knew his head coach secretly loved it.

Asked Tuesday to reflect on his Olympic experience, Tatum said he frequently spoke with Celtics Coach Joe Mazzulla during his time in Paris, which included two DNPs and the second-fewest minutes of any member of Team USA.

“Motivation, I guess you could say that if you want to simplify it,” Tatum said during Celtics Media Day. “In real time, it was tough. I talked to Joe a lot. Joe was probably the happiest person in the world that I didn’t win Finals MVP and I didn’t play in two games of the Olympics. That was odd, but if you know Joe, it makes sense.”

The reason for that is obvious. Mazzulla is laser-focused on leading the Celtics to another NBA championship this season, and his best player now has something more to prove after being an afterthought on the Olympic squad.

“Did I need any extra motivation coming into the season? No, I’m not gonna give anybody in particular credit that they’re motivating me to come into the season,” Tatum continued. “It was a unique circumstance, something I haven’t experienced before in my playing career. But I’m a believer that everything happens for a reason.”

Tatum focused during the leadup to training camp on fixing his jump shot, putting in extra work with trainer Drew Hanlen. He struggled as a shooter during both the NBA playoffs (31.4% from outside the restricted area) and the Olympics (0 for 16 on jump shots) and wants to clean up “a few mechanical things” before the new season tips off next month.