Celtics center Robert Williams to undergo knee surgery this week

Celtics center Robert Williams tore the meniscus in his left knee in March.Jim Davis/Globe Staff

Celtics center Robert Williams is scheduled to undergo left knee surgery this week that will likely sideline him for 4-6 weeks, according to a league source.

The source also said that Williams, who tore the meniscus in his left knee in March, was experiencing general soreness as he ramped up workouts while preparing for the start of training camp next week.

Williams’s March surgery sidelined him until Game 3 of the opening-round playoff series against the Nets. He missed four games of the conference semifinals against the Bucks while dealing with lingering soreness, and he sometimes appeared hobbled during the rest of the playoffs. But the Celtics insisted it was simply a matter of pain tolerance.

After the Celtics lost to the Warriors in the NBA Finals, president of basketball operations Brad Stevens said he did not anticipate any players requiring offseason maintenance surgeries. And if it was believed that Williams would need a procedure, it certainly would have been completed much earlier than this.

“Rob’s knee, as he went through the whole course of the playoffs, especially when we were playing every other day in the second round and in the third round, it was very much how he reacted day to day within the 48-hour window,” Stevens said in June. “He felt much better in the Finals, and obviously played pretty well in the Finals. But he needs to take some time. Both our doctors and others that we have consulted with have said the same thing. There’s certainly no risk being out there. But when it’s over, you need to take a few weeks for it to completely settle down, and so that you’re not putting as much load on it as we do as you go through the playoffs.”

Williams’s surgery will cause him to miss the preseason and likely the start of the regular season. The Celtics open the regular season against the 76ers on Oct. 18.

Williams, who turns 25 next month, averaged 10 points, 9.6 rebounds, and 2.2 blocks per game last season and was a second-team All-Defense selection. His absence will leave Boston’s frontcourt thin, with 6-foot-10-inch forward Danilo Gallinari already out for the season after tearing his ACL while playing for Italy in a FIBA World Cup qualifier this month. A league source said that Gallinari is expected to undergo surgery later this week.

What does the future hold for Robert Williams?Jim Davis/Globe Staff

In July, the Celtics traded third-string center Daniel Theis to the Pacers in the deal that brought Malcolm Brogdon to Boston. With Robert Williams out, coach Ime Udoka could turn to a smaller lineup that features Grant Williams at power forward and Al Horford at center. Also, Stevens has expressed confidence in veteran center Luke Kornet, and after waiving forward Bruno Caboclo on Tuesday, the Celtics have two empty camp roster spots, so they could look to add another center in the coming days.

A league source said that the decision to part ways with Caboclo, a journeyman who was signed to a camp deal last month, was unrelated to Robert Williams being sidelined for an extended period.