Dave Kearney is unavailable for the remaining games in Leinster's European Champions Cup pool campaign.
The winger has had to undergo surgery to rectify an ankle injury.
Kearney will be ruled out of action for six weeks according to Leinster coach Stuart Lancaster, having undergone surgery on the advice of a specialist.
The province are hopeful, however, that Jonathan Sexton will be back from his hamstring troubles in time for the New Year's Eve meeting with Ulster.
The province's depth at out half is being tested, with the eight week ankle injury suffered by newly capped Irish international Joey Carbery.
Fellow youngster Ross Byrne has been filling in for the absences, including in the Champions Cup against Northampton.