O'Neal didn't travel with the team for Monday night's game at Utah, and the reserve is considered day to day. An MRI exam confirmed the injury.
He was hurt early in the fourth quarter of Golden State's 102-88 victory over the Jazz on Saturday night in Oakland when he slipped along the baseline.
The Warriors signed free-agent center Dewayne Dedmon from the franchise's NBA Development League affiliate in Santa Cruz to help fill O'Neal's absence.
Dedmon played in five preseason games with Golden State, averaging 3.4 points, four rebounds and one block in 10.3 minutes. He was waived on Oct. 25.
O'Neal is in his 18th season in the NBA. He has battled numerous knee injuries throughout his career, though the most serious have been to his left knee.