Jamie Cudmore won't play again this season after suffering a concussion during training this week for Clermont.
The Canadian second row hadn't featured since the Champions Cup final loss at Twickenham but was hoping to make his return in the Top 14 semi-final against Toulouse.
However he suffered another knock to the head on Monday and will now be out of action both this week and in case of a potential final next weekend.
"He took a knock on Monday at training and as he'd already suffered a concussion in the European Cup, that's now two," explained Clermont coach Franck Azéma.
"We are going to respect the waiting periods and as a result he won't be with us (for the end of season). He will undergo further tests with a neurologist."