Martin O'Neill knows he is under increasing scrutiny as Celtic manager after alarming successive losses - but insists Sunday's Old Firm derby isn't a must-win fixture. Celtic are looking to bounce back after insipid defeats to Rangers in the League Cup last eight and Ferencvaros in the Europa League opener. O'Neill's side top the Premiership by five points from Rangers, who have won five successive games before the derby which precedes a three-week hiatus for international action.