Ulster number eight Nick Williams is expected to be out for eight weeks after sustaining medial ligament damage to his left knee.
Williams was a try-scorer in Ulster's Heineken Cup Round Five win over Glasgow Warriors on Friday, before being substituted in the 62nd minute.
The former Blues and Aironi number eight has been in impressive form this season for the Irish side, with seven tries including three in his last three matches.
Outhalf Paddy Jackson was assessed yesterday for an ankle injury he picked up against Glasgow, as was winger Craig Gilroy who suffered a bang to the head on Friday night.
Jackson's ankle is responding well to physiotherapy treatment, while Gilroy is following the necessary return-to-play protocol.
It is hoped that both players will be available for selection for this weekend's Heineken Cup game against Castres Olympique.