Williams suffered a sprained left knee during Sunday's win over the Timberwolves, Tim Bontemps of ESPN reports.
Williams has been dealing with tendinopathy in his right knee on and off this season, so this does not appear to be related to that injury. The big man exited Sunday's blowout win late in the third quarter and was ruled out for the remainder of the contest shortly after. With the Celtics playing on a back-to-back, it's quite likely that Williams will be held out of Monday's game at Toronto. As of now, though, no official decision has been made.