Although the injury is not as bad as first feared - it was thought that he would be out for up to a year - the popular defender requires surgery to establish the full extent of the damage.
The news comes as a blow to all concerned at Millwall and will seriously the club's play-off hopes. Muzzy was an integral part of The Lions defence and will be sorely missed.
On a more personal note, the injury also dashes the Australian international's hopes of joining an elite band of players that had played in both the English and Scottish FA Cup finals.