The surgery, which the team said was done to "remove loose bodies and address swelling in his left knee," is the second one Williams has had on that knee this calendar year.
On March 30, Williams underwent a partial left knee meniscectomy after suffering a torn meniscus. He then returned to the court for Game 3 of Boston's first-round sweep of the Brooklyn Nets, but went on to have recurring soreness and swelling in the knee throughout the playoffs that caused him to miss a total of seven of the Celtics' 24 postseason contests on their way to losing to the Golden State Warriors in six games in the NBA Finals.
After Game 6 of the Finals, Williams said that the only treatment he would need for his knee was rest, and Celtics president of basketball operations Brad Stevens reiterated the same message at his own news conference after Boston's season had concluded.
A return to basketball activities in eight to 12 weeks would keep Williams -- one of the most impactful defensive players in the NBA, and a key part of Boston's suffocating league-leading defense -- out until at least Thanksgiving, and possibly until after Christmas.
Williams, who will turn 25 next month, averaged 10 points, 9.6 rebounds and 2.2 blocks in a career-high 61 games for Boston last season, his fourth in the NBA.
The Celtics also announced Danilo Gallinari underwent surgery on his own knee injury, a torn ACL he suffered back in late August while playing in a FIBA World Cup qualifier for Italy. Gallinari, who signed a two-year, $13 million contract with Boston in July as a free agent, was expected to provide a significant boost to bench scoring this season alongside guard Malcolm Brogdon, whom Boston acquired in a trade with the Indiana Pacers.
Last season with Atlanta, Gallinari averaged 11.7 points per game and shot over 38.1% from 3-point range. In 728 career games, Gallinari, 34, has averaged 15.2 points per game and shot 38.2% from 3-point range.