Jack McGrath has been ruled out for "the next few weeks" after picking up a knee injury.
The Leinster and Ireland player will miss several Pro 14 matches. The Champions Cup defence begins on 12 October in the RDS against Wasps.
Dan Leavy and Sean O’Brien remain unavailable for the province because of shoulder problems.
Leo Cullen’s side were beaten 23-21 by the Scarlets on Saturday.
