However, Boston coach Ime Udoka clarified that the soreness doesn't stem from his surgery last month to repair the meniscus in the knee. Instead, scans revealed a small bone bruise.
"He got his knee scanned and everything structurally is good." Udoka said after shootaround 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 it's just that's where the swelling and soreness came from."
The Celtics haven't had Williams for the past two games in the best-of-seven series against Milwaukee and split those contests. They trail the Bucks 3-2 heading into Game 6.
When asked if the issue occurred 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 Games 3 and 4, coming off the bench in both of those matchups during the sweep. He returned to the starting lineup against the Bucks.
Grant Williams has started both games in place of Robert Williams but the team has also found some success with smaller lineups. Grant Williams has played in 56 minutes in the past two games while guard Derrick White has played 66 minutes over that span.
White said he talked to Robert Williams, who 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."