Ulster lock Dan Tuohy has a torn calf muscle and will be out for up to four weeks.
Tuohy was injured in Ulster's narrow defeat to Northampton Saints at Ravenhill on Saturday night and went for scans on Monday which revealed a tear.
Paddy Jackson sustained a dead leg in the same match. He is responding well to treatment and will likely be available for selection for the Pro12 game against Leinster on Friday evening.
No date has yet been set for Tommy Bowe's knee surgery but it is expected that it will take place before the end of this week.