Ulster second-row Dan Tuohy will be out for up to four weeks with a torn calf muscle.
The Ireland international, 27, suffered the injury during Saturday’s loss to Northampton Saints at Ravenhill.
The RaboDirect PRO12 leaders are also missing captain Johann Muller who broke his arm last week.
A statement from Ulster Rugby also confirmed that Tommy Bowe is likely to have knee surgery this week.
Out-half Paddy Jackson sustained a dead leg in that game but is expected to be fit for Friday’s game against Leinster.