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Perkins: Celtics are a second-round exit without Robert Williams III (1:13)

Robert Williams III's injury absence is a big loss for the Celtics. (1:13)

8:27 AM ET

According to sources, Boston Celtics center Robert Williams III is expected to return within four to six weeks after having surgery on his left knee.

Williams suffered a tear in his rotator cuff on Sunday night, but the extent of the injury allowed for a procedure that will not require him to be out for a long time.

With six games left in the regular season, Boston is third in the Eastern Conference and only one game out of first place.

Over the past two months, Williams has been an immense part of the NBA's best defense and hottest team. He is the closest defender among 169 players who have defended 500 or more shots. Williams has held opponents under 40% shooting.

Williams, who signed a new four-year, $52 million extension in the off-season, has averaged 10 points, 9.6 rebound and 2.2 blocks and is shooting 73.6% from the floor.