Celtic captain Stephen McManus was today ruled out of Saturday's Old Firm derby with a knee injury. The problem forced the Scotland defender, 26, from the field during last weekend's 3-1 Clydesdale Bank Premier League win at Aberdeen. That was McManus' first outing since he succumbed to an ankle injury in the 1-1 draw at Hearts three weeks earlier. Celtic are already without midfielder Scott Brown for their crunch clash at Ibrox, with the newly-crowned PFA Scotland Player of the Year in the middle of a two-match ban.