Pau suffer another huge blow
He will be out for six to nine months.
The 32-year-old former England international, Pau's most used player this season, underwent tests on Monday which confirmed the gravity of the injury.
The news continues a bad couple of weeks for Pau.
The semifinal defeat in Wales knocked Pau out of the European Challenge Cup and followed two consecutive losses in the Top 14 that have dropped the club to seventh and out of the play-offs.
In the second of the league losses, at home to struggling Agen, New Zealand fly half Colin Slade suffered a hamstring injury.
Pau are two points behind Castres in the last play-off berth and visit leaders Montpellier this weekend before hosting Armitage's former club Toulon, who are fourth, in the final round.