Lee suffered the tear during Game One of the Warriors' first-round playoff series with the Denver Nuggets on Saturday when he fell to the court after contact with JaVale McGee on a drive to the basket in the fourth quarter.
After undergoing X-rays on Saturday, Lee had an MRI on Sunday that determined the severity of the damage, CBS and USAToday reported. The injury originally was diagnosed as a strain.
Losing Lee is a blow to the Warriors' hopes in the postseason. The 29-year-old is Golden State's second-leading scorer this season at 18.5 points per game and its top rebounder with an 11.2 average in 79 regular-season game.
Meanwhile, veteran point guard Steve Nash was back in the starting lineup for the Los Angeles Lakers on Sunday against the San Antonio Spurs in the opener of their NBA Western Conference play-off series in San Antonio.
The 39-year-old Nash, a two-time NBA most valuable player, had missed the final eight games of the regular season because of a hamstring injury.
Also, Chicago Bulls Derrick Rose has not stated whether he will return for the play-offs, and the Bulls have not completely dismissed the idea of a return.
"Most likely out, but you never know," Chicago coach Tom Thibodeau told CSN Chicago. "The playoffs are stretched out too, so you have to factor that in too.
"So, who knows another week from now where he is so, you always want to have that possibility open."