I think Silva catches him with something. . . GSP has never seen anything close to the striking of a guy like Silva. . Not to mention the size difference - while GSP would probably still be physically stronger of the two, it wouldn't be the strength difference he's used to.
Silva catches him with a nasty knee, or up kick or something, and GSP would be helpless to the onslaught. . . Matt Serra aired his ass out. . .