Gelabale was injured in practice on Tuesday. He underwent an MRI on Wednesday and team physicians confirmed the injury Thursday morning. He will require surgery.
Gelabale played in 39 games this season, also briefly playing with Idaho in the NBA Development League. Gelabale averaged 4.3 points and 1.5 rebounds in about 12 minutes per game for the Sonics, and had played better since being recalled from Idaho. He had a four game stretch last month scoring in double figures, including a career-high 21 points against the Lakers.
Gelabale, in his second full season in the NBA, is the second player Seattle has lost this season to knee surgery, joining oft-injured center Robert Swift.