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.
As head coach Stuart Lancaster polishes the axe he will soon wield over his World Cup training squad, Exeter Chiefs coach Rob Baxter has backed Jack Nowell, Henry Slade and Luke Cowan-Dickie to be given a stay of execution.
Italy Under-20s held off a late Samoa blitz to retain their place in the World Rugby U20 Championship on Saturday and head coach Alessandro Troncon believes the experience will have served as a valuable lesson for his young troops.