"I don't know," Popovich said when asked when Gay might be back in the lineup. "I just know he's not there tonight. Right now, that's all that matters."
Jakob Poeltl will start in Gay's place against the Celtics, Popovich said.
Gay, who is averaging 13.7 points, 6.5 rebounds and 2.4 assists in 30 starts this season, entered the locker room Monday with a black brace on his left wrist.
He said the injury happened when he fell on the wrist during Saturday's game against the Clippers, but he "didn't really feel it until after the game" and that he "couldn't even move it."