France centre Aurelien Rougerie could struggle to be fit in time for the World Cup starting in September after an operation on a dislocated ankle.
He suffered the injury when his club Clermont took on Toulouse in France's domestic Top 14 league on Saturday.
Clermont said in a statement: "A six-week spell of bed rest will be followed by physiotherapy on his ankle.
"His period out of action will depend on how quickly the scar heals and how well his rehabilitation goes."
Clermont said the most precise it could be was that its club skipper would be out "for several months".
France's first World Cup match is on 10 September, against Japan in Auckland, but coach Marc Lievremont is due to name his preliminary squad on Wednesday.
A specialist wing, Rougerie was converted to centre at international level by Lievremont last November and had been considered a certainty for World Cup squad selection.
Among other French injury concerns, Rougerie's club-mate, prop Thomas Domingo, has a serious knee injury while Stade Francais hooker Dimitri Szarzewski has an Achilles tendon problem and centre Maxime Mermoz a long-standing shoulder injury.