Ulster captain Iain Henderson and James Humes face lengthy lay-offs after undergoing surgery on Monday.
Ireland lock Henderson had toe surgery and is expected to be sidelined for approximately three months.
Centre Hume has a knee injury and “will enter a period of rehab for the coming months”.
Ulster host Irish rivals Leinster on Saturday in their penultimate United Rugby Championship game of the season.
Luke Marshall (concussion) and Rob Baloucoune (hamstring) picked up injuries in training and will miss the Kingspan Stadium encounter.
Tom O’Toole and Sean Reffell are doubts for the derby with their availability to be assessed through the week.