Ulster wing Tommy Bowe faces a lengthy spell out after suffering knee ligament damage in the Heineken Cup defeat by Northampton at Ravenhill on Saturday.
Ulster said Bowe "sustained significant lateral ligament damage to his right knee" after landing awkwardly.
The length of the lay-off he faces will be determined after the Ireland player undergoes surgery next week.
Meanwhile second row Dan Tuohy has been ruled out for four weeks after suffering a torn calf muscle.
The news will come as a major blow to coach Mark Anscombe, who is already without Tuohy's second row partner Johann Muller for a number of weeks.
Ulster medical staff are continuing to monitor Paddy Jackson, who sustained a dead leg against Northampton.
The fly-half is however responding well to treatment and is expected to be available for Friday night's Pro12 game against Leinster at Ravenhill.
Injuries to lock Dan Tuohy (calf) and fly-half Paddy Jackson (dead leg) will also be assessed on Monday.
Bowe, who was injured in the second half of the Pool Four game, returned to Ulster in the summer from the Ospreys.
The injury means the 28-year-old is also a doubt for the Six Nations campaign with Ireland.
"He will undergo surgery later this week to repair the damage," added Ulster on Sunday.
"At this stage it is not possible to put a timescale on how long his recovery period will be.
"A further update on this will be issued following the surgery, when the full extent of the injury will be known."
Meanwhile, Ulster's lead at the top of pool has been cut to three points after second-placed Castres defeated Glasgow 10-8 on Sunday.