Ulster and Ireland second-row forward Iain Henderson has undergone hip surgery and will be out of action for up to ten weeks.
It means he will miss the northern province's European Champions Cup last-eight encounter against Toulouse, which is scheduled to take place in France on Sunday September 20th.
Henderson will also be unavailable for the remainder of Ulster's Pro 14 campaign.
His absence will be a concern for Ireland coach Andy Farrell, with fellow international lock James Ryan also nursing a shoulder problem.
Ireland will finish their Six Nations campaign with back to back matches against Italy and France at the end of October.
Ulster return to action against Connacht in the Pro 14 League at the Aviva Stadium on Sunday August 23rd, and they could also be without attacking player Will Addison, who is rehabilitating from a back injury.
Greg Jones, Matthew Rea, Sam Carter and Matt Faddes have all returned to training after undergoing surgeries and are projected to be fit for the match with Connacht.
Angus Curtis is continuing his spell on the sidelines after he sustained a multi-ligament knee injury.
Andrew Warwick is due to undergo hip surgery next week.
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