Boston coach Ime Udoka said Williams was still experiencing soreness on Wednesday and the team decided to hold him out of the team's Game 5 110-107 loss to Milwaukee in its Eastern Conference semifinal series.
"The swelling went down and that's why we were hopeful for tonight, due to the swelling getting better over the last few days," Udoka said of Williams, who was listed as questionable. "But he had some soreness, so we'll be overly cautious coming off the surgery. Until it's pain-free, we'll rest him until he's ready."
Williams returned from a torn meniscus in his left knee in Game 3 of Boston's first-round playoff series victory over the Brooklyn Nets. He returned to the starting lineup for the first three games of the series.
Without Williams, the Celtics pulled away with a 116-108 win against Milwaukee on Monday night to even the best-of-seven series at 2-2.
Udoka said Grant Williams will again start in Williams' place.