"Unfortunate, especially for a guy that's worked so hard and put himself in position and put us in the position that we're in today. ... But the task is still at hand so we move forward," coach Mark Jackson said.
MRI results showed a torn right hip flexor.
"I knew last night when I did it," Lee said before the Warriors' practice Sunday. "I felt it pop. So I knew we were going to get the results we did this morning. I went run back on defence and had absolutely no sensation in my leg. It wasn't even painful as much as it was just (numb)."
Lee said doctors told him he likely won't need surgery, just a couple of months of rest and rehab. He said he hopes to be 100 per cent by mid-summer and come back stronger in camp.
The injury leaves the Warriors without a key producer down low. Lee led the NBA in double-doubles with 56 this season. He averaged 18.5 points, 11.2 rebounds and 3.5 assists.
"We can't replace him, and we know that," Jackson said. "And that's the thing we have to know right away. He's an All-Star power forward who's had an incredible year. But we have some guys on our bench that can do some things to cover up his absence. But we cannot replace David Lee."
Carl Landry stands to get the bulk of Lee's minutes with Draymond Green, Andris Biedrins and Festus Ezeli also in the rotation. Jackson might also go with a guard-heavy lineup at times.
Lee banged into JaVale McGee on a drive in the fourth quarter Saturday and winced as he landed on his right hip. After gathering himself, he shot both free throws, then struggled to run down court and needed assistance as he made his way to the bench.
"No dirty play or anything like that," Lee said. "I went up and when I landed, my foot caught, my weight kept all going forward and put all the pressure on that hip area. And I've been dealing with a sore back ... for the last week and a half, two weeks. And whether overcompensating for that had anything to do with it, I don't know."
Lee finished with 10 points and 14 rebounds in his first playoff game in eight NBA seasons.
"I've waited for eight years for this and to have it come to such an abrupt end with something that's out of my control is frustrating," Lee said. "But at the end of the day, our team has still got a good chance I think in this series and I'm going to put all my effort toward that."
Game 2 is Tuesday in Denver, and Lee will be there for his teammates.
"It's easy to kind of sit here and worry about yourself, but I've been a leader all year long and I'm going to continue to be one and guys need to see me be positive right now and I'm going to do my best to be that guy," he said.
Being a cheerleader or a coach, though, will be hard for him.
"I know it's really going to set in on Tuesday when we go out there and I'm not ready to play," Lee said.