The club's leading scorer was taken off against Doncaster with the problem and it was initially thought he would be out for four weeks.
But the fracture is worse than first suspected and the former Rangers player will miss the rest of the season, although there is hope that he will return for the play-offs.
Meanwhile, David Lucas will be out for around six weeks with his knee injury while Paul Smith joins MacLean on the sidelines for the rest of the season.