The Celtics will need to begin their NBA title defense without their star center.

Boston on Thursday announced Kristaps Porzingis is expected to be sidelined for the next five to six months after undergoing successful surgery to repair a torn retinaculum and dislocated posterior tibialis tendon in his left leg.

Based on that timeline, the earliest Porzingis could return to action is late November, with his absence potentially stretching until Christmas. That means he’ll miss at least the first month of the Celtics’ 2024-25 season, which likely will begin in late October.

Porzingis suffered the injury — which the team called “rare” — in Game 2 of the NBA Finals against the Dallas Mavericks. The 7-foot-2 big man sat out Games 3 and 4 before returning to play 16 minutes off the bench in the Game 5 clincher.

Head coach Joe Mazzulla said Porzingis, who missed 10 games earlier in the playoffs after suffering a calf strain in the opening round, overruled Boston’s medical staff and insisted on playing in the postseason finale.

“I was like, listen, I’m going to give it everything I have,” Porzingis said after the game.

The Celtics are set to return nearly every key contributor from their championship squad, but their frontcourt depth behind Porzingis and 38-year-old Al Horford is unsettled. Backups Luke Kornet, Xavier Tillman and Oshae Brissett all are unrestricted free agents, and Neemias Queta has a team option in his contract for the upcoming season.

Horford is in great shape for his age and played well as a playoff starter with Porzingis sidelined, but the Celtics will want to avoid overloading the 18th-year veteran. So, assembling the proper group behind him to withstand Porzingis’ impending absence should be Boston’s top offseason priority.

The Celtics could look to fill that void by re-signing some of their internal free agents or scoring the market for a cheap replacement. Boston’s salary cap situation will prevent it from making any splashy veteran additions, but there’s room for a budget signing or two. Two-time ex-Celtic Daniel Theis, most recently of the Los Angeles Clippers, would be one logical target.

Boston also could look to address this need in the second round of the NBA draft, though president of basketball operations Brad Stevens said any rookie will have a hard time cracking the Celtics’ rotation.

The Celtics used their first-round pick (No. 30 overall) to select sharpshooting Creighton guard Baylor Scheierman on Wednesday.