"#NEBHInjuryReport Robert Williams underwent a successful arthroscopic procedure to remove loose bodies and address swelling in his left knee," the Celtics posted on their verified Twitter account. "Williams is expected to return to basketball activities in 8-12 weeks."
Earlier this week, ESPN's Adrian Wojnarowski reported that Williams was expected to miss all of training camp and the preseason due to surgery.
Williams had previously underwent meniscus surgery in March and returned during Celtics' postseason run, which ended in a Game 6 loss to the Golden State Warriors in the NBA Finals.
"Robert Williams’ procedure will keep him minimally out of the Celtics training camp and preseason, but the clean up on his left knee is expected to have him back in the lineup early in the regular season, sources said," Wojnarowski tweeted at the time.
Williams returned to the Celtics' lineup during Game 3 of the first-round of the Eastern Conference Playoffs, playing 16 minutes during Boston's 109-103 win against the Brooklyn Nets.
The 24-year-old was limited during the Eastern Conference Semifinals, but appeared in five of the Celtics' seven Eastern Conference Finals games against the Miami Heat and all six games in the NBA Finals against the Warriors.
Williams started in all 61 games he appeared in for the Celtics during the 2021-22 NBA season before experiencing a knee injury during the team's 134-112 victory against the Minnesota Timberwolves on March 27.
Williams averaged 10.0 points, 9.6 rebounds, 2.0 assists and 2.2 blocks per game, all of which were career highs, during the 2021-22 regular season, his fourth as an NBA player.
The Celtics also confirmed that forward Danilo Gallinari "underwent successful left knee revision ACL repair" on Friday.
"Further updates regarding Gallinari's recovery will be provided as appropriate," the team tweeted.