However, Boston coach Ime Udoka clarified that the soreness is not stemming from his surgery last month to repair the meniscus his Williams' left knee. Instead, scans revealed a small bone bruise.
"He got his knee scanned and everything structurally is good." Udoka said after shootaround on Friday morning. "Looks fine. The swelling went down but he still had some soreness and pain. Revealed that he has a small bone bruise from collision he took in Game 3. Structurally he's fine from the surgery is just that's where the swelling and soreness came from."
The Celtics haven't had Williams for the last two games in the best-of-seven series against Milwaukee and split those games. They trail the Bucks 3-2 heading into Friday's Game 6.
When asked if it was when Williams collided with Bucks forward Giannis Antetokounmpo in Game 3, Udoka said "Yes," but quickly added that "Giannis ran into him" hiding a wry smile underneath his mask before taking the next question.
Udoka added the current injury is what has caused Williams' absences.
"There's no problems with the surgery at all," Udoka said. "It's just that specific hit that he took."
Williams returned from the knee surgery in the first-round series against the Brooklyn Nets in Game 3 and 4, coming off the bench in both games of that sweep. He returned to the starting lineup against the Bucks.
Grant Williams has started both games in place of Williams but the team has also found some success with smaller lineups. Grant Williams has played in 56 minutes in the last two games while guard Derrick White has played in 66 minutes over that span.
White said when he talked to Robert Williams and said he's feeling good but knows that he's just waiting to get the green light to rejoin his squad on the court.
"Rob's special," White said. "We've been saying that all year. What he does, no one else in the league does. It's always great to have him out there. When he's out there, he always does great for us."