An MRI on Friday revealed the extent of Ibaka's injury, a grade 2 sprain, which he suffered in the third quarter of a series-clinching Game 6 victory against the Los Angeles Clippers on Thursday night.
"The severity of the injury is not long-term," Thunder general manager Sam Presti told reporters in a conference call. "But the the timing of the injury and how deep we are in the playoffs is unfortunate."
Ibaka averaged career-highs of 15.1 points and 8.8 rebounds during the regular season and led the NBA in total blocks (219) for the fourth consecutive season. He averaged 12.2 points, 7.3 rebounds and 2.2 blocks in 13 postseason games.
The Thunder will likely replace Ibaka with veteran Nick Collison, who averaged 4.2 points and 3.6 rebounds in 16.7 minutes per game in the regular season.
OKC begins the Western Conference finals on Monday against the reigning West champion Spurs in San Antonio. The Spurs start 14-time NBA All-Star Tim Duncan, who is averaging 15.8 points and 8.3 rebounds in the postseason, at power forward.
This is the second straight postseason Oklahoma City has lost a key starter. Last year, a knee injury sidelined point guard Russell Westbrook in the first round of the playoffs.