Ulster and Ireland lock Iain Henderson is facing a lengthy spell on the sidelines because of a hip injury.
The 22-year-old will have an operation on the injury and is a doubt for Ireland's November Test series.
Henderson impressed on Ireland's successful tour of Argentina in June and played in four of the five Six Nations games earlier this year.
Ulster full-back Jared Payne also has a hip injury and is expected to be out for at least two weeks.
The Irish province host Zebre on Friday before an away encounter against Cardiff Blues a week later.
New skipper Rory Best is expected to be available for Friday night's clash with the Italian outfit, but doubts remain over the availability of Tommy Bowe, Darren Cave, Paddy Jackson and Luke Marshall.
Zebre suffered a 26-41 home defeat to Cardiff Blues in their opening fixture last weekend.
Henderson, who can also play in the back-row, represented Ireland at Under-19 and Under-20 level before making his senior debut against South Africa in November 2012.
He has won 12 internationals caps although he could now miss the November Tests against South Africa, Georgia and Australia.