Ulster fly half Paddy Jackson faces up to six weeks out of action.
The 22-year-old picked up a shoulder injury against Ospreys.
In a statement, Ulster said that he had "suffered an AC joint sprain at his right shoulder in Friday night's win over Ospreys at Kingspan Stadium.
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"He is expected to be out of rugby for up to six weeks."
Jackson was left out of the latest squad for the November Internationals, although Ireland head coach Joe Schmidt said he remains on the radar.
Irish Times rugby correspondent Gerry Thornley also felt that being left out was not a death knell for his international hopes.