Clermont and France international wing Aurelien Rougerie will be out of action for at least three months after sustaining a hernia injury.
The ASM captain, who was joint top try scorer in the Heineken Cup last season with five tries, looks unlikely to feature in any of the French championship finalists' opening four Heineken Cup Pool 1 games.
The 2007 European Challenge Cup winning captain is expected to under go shoulder surgery in the coming weeks and will also be unavailable to France for their November tests against Australia, Pacific Islands and Argentina.
The injury news is another blow for ASM who have suffered two losses in three outings in the new Top 14 season.
ASM's European campaign will get under way without their influential captain when they welcome Sale Sharks on Saturday, 11 October.
Finals Edition: Listen to the best guide to the European Rugby Cup Finals weekend in Dublin, which sees Leinster Rugby take on Stade Francais Paris at the RDS on Friday night in the Amlin Challenge Cup Final and then ASM Clermont Auvergne tackle Toulon in an all-French affair at the Aviva Stadium on Saturday.