MILWAUKEE — Boston forward/center Robert Williams missed his third straight game with an injured left knee as the Celtics tried to keep their season alive Friday night in Game 6 against the Milwaukee Bucks.

Williams had been listed as questionable on the Celtics’ injury report. Celtics Coach Ume Idoka said at the team’s post-shootaround availability Friday that Williams hurt the knee during a collision with Milwaukee’s Giannis Antetokounmpo in Game 3.

“Once the swelling went down, we found out he had a bone bruise,” Udoka said Friday before the game. “There’s a high pain threshold there. It limits his movement when he bends a certain way.”

Udoka said he didn’t believe this was a long-term issue.

The Bucks lead the Celtics 3-2 in the Eastern Conference semifinal, with the winner set to face the Miami Heat. If needed, Game 7 would be Sunday in Boston.

Bucks forward Khris Middleton was out for a ninth consecutive game with a sprained medial collateral ligament in his left knee. Bucks coach Mike Budenholzer didn’t offer any indication on whether the three-time All-Star might be available the next time Milwaukee plays after Friday.

“He’s doing well,” Bucks Coach Mike Budenholzer said before the game. “He’s making progress. We’re optimistic, but no new update.”