Flanker Sean Reidy will miss Ulster's PRO14 run-in after shoulder surgery ruled him out for three months.
With the final of this year's competition taking place during the last weekend of March, such a timescale would leave him struggling to play any further part in this campaign.
The 31-year-old was enjoying a rich vein of form either side of the sporting shutdown last year but picked up the injury in the European defeat to Gloucester last month and his absence will be keenly felt in Dan McFarland's back-row.
There was more encouraging news on Jacob Stockdale after the Irish star limped out of Saturday's victory over Munster.
The knee injury he sustained in the final minutes of the interpro is not serious and his availability for Friday's key show-down with Leinster in the RDS will be assessed through the remainder of this week.
Jack McGrath, Iain Henderson, Luke Marshall, Rob Baloucoune, Rob Lyttle, Will Addison, Louis Ludik and Angus Curtis all remain on the sidelines with no updates issued on their prospective return dates.