Ulster will be without flanker Stephen Ferris for their Heineken Cup quarter-final match against Northampton Saints on April 10.
Ferris, who sustained a left knee injury during Ulster's match against Aironi in January, is to undergo arthroscopic surgery following an assessment by his surgeon.
"It is hoped that following this procedure a definitive diagnosis and prognosis for return to rugby will be reached," a statement from Ulster Rugby said.
Meanwhile Ian Humphreys and BJ Botha return from injury in a 35-man Ulster squad named yesterday ahead Friday's Magners League clash against the Scarlets.