Ireland's Stephen Ferris has been ruled out of action for up to six weeks.
The Ulster back row player sustained the injury on international duty against Argentina in June and, although he travelled to France with the Ireland squad for the World Cup, he has been unable to train since his return.
Ulster confirmed that Ferris' injury had flared up, and the medical staff advised that he rest for four-six weeks.
Ferris will miss the opening European Cup pool game against Gloucester and next week's trip to Bourgoin.
He could come back in contention for round three or four when Ulster face the Ospreys in back-to-back fixtures.