Ulster captain Johann Muller has been ruled out for four to six weeks after he suffered a calf injury during their 39-21 PRO12 victory over Cardiff Blues.
The South African veteran had a scan on Monday that confirmed he has a torn calf muscle.
"Johann, at this stage, I'd say is pretty doubtful. He won't be around next weekend and with Dan (Tuohy) going (into camp with Ireland), that hurts us a bit," said Ulster head coach Mark Anscombe after the recent game, with his fears confirmed.
"But Lewis (Stevenson) came on and did a good job and we're fortunate to have someone of his capabilities and experience. Now we've just got to find someone else to lock up with him next week."