Ulster's Craig Gilroy and Will Addison have been ruled out of Saturday's Champions Cup quarter-final against Leinster at the Aviva Stadium.
Both Irish internationals will miss the game due to back injuries.
The Northern Province are hopeful that Louis Ludik and Marty Moore can play.
Ludik hurt his knee during the weekend's Pro14 win over the Southern Kings and will continue to be assessed in the coming days.
Former Leinster prop Moore is following return-to-play protocols having sustained a concussion in the same match.