Petta had a number of injuries during his time at Celtic; Celtic won the domestic treble in 2001 but after picking up an injury in the 2001 Scottish League Cup Final Petta missed the 2001 Scottish Cup Final. He also missed the 2003 UEFA Cup Final as a result of injury. Due to the injuries and falling out of favour with manager Martin O'Neill Petta only made one appearance for the Celtic first team between November 2002 and his departure from the club in late 2004; his sole appearance coming against MTK Hungaria in a UEFA Champions League qualifier.