Ireland international scrum-half Peter Stringer could be out of action for up to a month after sustaining knee ligament damage.
The 27-year-old suffered the injury during a training session in Limerick while on duty with Munster.
Stringer will miss the province's opening two European Cup fixtures against Sale and Castres but should be back in time for Ireland's autumn Test opener against New Zealand on November 12.
That would be a boost for Ireland coach Eddie O'Sullivan, who is certain to be without Stringer's regular international deputy Guy Easterby for the majority if not all of the November run of Tests.
Leinster scrum-half Easterby broke his jaw against Ulster and is scheduled to be out of action for up to six weeks.
Easterby's injury could be a blessing in disguise for Ulster scrum-half duo Kieran Campbell and Isaac Boss, the recently arrived Irish-qualified New Zealander, who both look certain to step up to internationals in the near future.