An MRI confirmed that Lee has a partially torn patellar tendon, his agent Mark Bartelstein told ESPN's Ramona Shelburne. He will be re-evaluated in a few days to determine whether surgery is needed, Bartelstein said.
Lee suffered the injury during the Spurs' Game 3 loss to the Warriors. Popovich said earlier Sunday, however, that he didn't think the injury was as serious as first believed.
"He's probably not gonna play, but I don't think it's serious," Popovich said. "He did an MRI, and we haven't gotten the results yet. But hopefully he just got like a bruise or something like that. It didn't really swell up, it just got stiff. So he'll probably be out."
Lee suffered the injury in the first quarter of San Antonio's 120-108 loss on Saturday night and did not return. He left the game with 2:57 remaining in the first quarter, after he scored on a layup contested by Draymond Green.
Lee was fouled on the play, but he was unable to shoot the ensuing free throw attempt because of the injury. He winced upon landing after the layup, and he was down on the floor for a couple of minutes near the tunnel leading out toward the locker rooms.
After athletic trainers helped Lee to his feet, he hobbled toward the locker room before staffers took over and rushed him down the hall in a wheelchair.
Lee became the club's third player to suffer an injury significant enough to take him out of action. The Spurs were already short-handed without star forward Kawhi Leonard, whom Popovich held out because of a sprained left ankle.
Leonard's injury took place after the club already had lost Tony Parker in the Western Conference semifinals because of a ruptured quadriceps tendon.
The Warriors lead the best-of-seven series, 3-0.