Boston Celtics center Robert Williams will miss four-to-six weeks following an arthroscopic procedure on his left knee, sources told ESPN's Adrian Wojnarowski.

Williams will reportedly undergo the procedure Thursday. He's expected to return early in the regular season but will miss training camp and the preseason, Wojnarowski adds.

The 24-year-old underwent meniscus surgery in March before returning for the playoffs, averaging 7.7 points, 6.2 rebounds, and 2.2 blocks in 17 postseason games.

Boston is already without veteran forward Danilo Gallinari after the 34-year-old tore his ACL while playing for Italy during World Cup qualifiers earlier this offseason.