Manchester United have confirmed that defender Nemanja Vidic broke his collarbone during Saturday's 4-1 win over Blackburn.
Vidic may not play again this season
United boss Sir Alex Ferguson said after the match that the Serbian was set to be out for four to five weeks.
United were already without the injured Mikael Silvestre and Gary Neville.
Wes Brown, who had been deputising for Neville at right-back, switched to central defence to partner Rio Ferdinand when Vidic was injured.
United may have to stick with that formation in Wednesday's Champions League quarter-final, first leg against Roma.