Ulster No.8 Nick Williams will be sidelined for eight weeks after suffering a knee injury in the European Cup win over Glasgow Warriors at Ravenhill on Friday.
An assessment on Monday morning confirmed that he sustained medial ligament damage to his left knee and is expected to be out for up to two months.
Prop Adam Macklin damaged his knee playing for the Ravens against Bristol at the weekend. He will be subject to an ongoing conservative management programme and will be reviewed in due course.
Flyhalf Paddy Jackson was assessed on Sunday for an ankle injury he picked up against Glasgow, as was wing Craig Gilroy who suffered a bang to the head on Friday.
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 game against Castres.