Via a Thunder Press Release:
Oklahoma City Thunder forward Nick Collison underwent successful arthroscopic surgery on his left knee. Collison is expected to miss 4-6 weeks before returning to normal offseason activities.
Nick Collison didn't miss any time over the past season due to his knee, so this is rather unusual. The few minor injuries that he did have concerned his hip and quadriceps. However, Collison has had arthroscopic surgery before, undergoing the procedure on his right knee back on May 13th of 2010. The last surgery concerned his left knee and he was able to return to normal production during the next season, so this should be seen as nothing more than precautionary.