Aurelien Rougerie will miss Clermont Auvergne's final two European Champions Cup group matches after sustaining a knee injury.
The veteran back, capped 76 times by France, picked up the problem in Sunday's 31-27 home defeat to Castres.
Clermont released an update on Tuesday, confirming he would be out for a month due to a partial ligament strain.
He is likely to miss Clermont's Top 14 fixtures against Racing 92 and Montpellier, which fall either side of Champions Cup clashes with Northampton Saints and the Ospreys.
Clermont sit top of Pool 2 having won all four games this season, holding a five-point lead over the Ospreys.