The Celtics don’t know exactly when Kristaps Porzingis will be healthy enough to return to the court this season.

But they’re encouraged — and, according to Brad Stevens, even a little surprised — by the progress he’s made in his recovery.

“Kristaps obviously had the surgery that was well-documented after the year,” Stevens said Tuesday at Celtics media day. “He seems to be recovering well. He’s very optimistic. I don’t know that we’re interested in putting a timeline on it, 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 underwent surgery in late June to repair what the team called a “rare” lower-leg injury suffered during the NBA Finals. The Celtics estimated a recovery period of five to six months, meaning the center would miss at least the first month of the regular season, which begins Oct. 22.

In an interview with ESPN’s Ramona Shelburne, Porzingis, who spent the offseason rehabbing in his native Latvia, said his “expectation” is to return “sometime in December.”

“But I’m feeling really good,” he told ESPN, “and I’m working towards hopefully playing earlier than that.”

Porzingis enjoyed a strong debut season with the Celtics, averaging 20.1 points, 7.2 rebounds and 1.9 blocks per game while shooting a career-best 51.6% from the floor. Boston played well without him, however, going 31-6 in games the 29-year-old missed, including a 10-2 mark in the postseason.

Veteran Al Horford is expected to be Boston’s primary starting center in Porzingis’ absence, with reserves Luke Kornet, Xavier Tillman and Neemias Queta also taking on larger roles.

Stevens revealed Tuesday that Kornet (wrist) and Tillman (knee scope) both underwent offseason surgeries but have fully recovered and will practice on Wednesday.

“I’m really encouraged by the health of our team,” Stevens said.