Ulster Captain Johann Muller suffered a neck injury during Sunday's match against the Blues at Cardiff City Stadium and the second row forward was helped from the field.
The South African has featured in all but one of Ulster's competitive matches to date this season, having been rested for the September trip to Connacht.
With the injury still fresh, an anticipated date to return to rugby has not yet been determined.
Back row Robbie Diack also took a knock during the game and has suffered a badly sprained ankle - it looks unlikely at this stage, that he will be fit to play against Treviso this weekend.
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