It is feared that Johnny Sexton will miss the remainder of Leinster's Champions Cup pool games - plus the first two Six Nations fixtures of Andy Farrell's new Ireland reign.
Sexton, 34, saw a specialist yesterday and it is believed that he has suffered a medial tear in his right knee that will force him to sit out the next eight to 10 weeks.
Ross Byrne will step into the out-half role for Leinster for Saturday's European tie with Northampton Saints, as well as the final pool encounters with Lyon and Benetton in January.
Byrne could come into the Test frame now as well if he performs well, although Munster hope that Joey Carbery is hoping to return to action against Leinster at the end of this month, having not played since the World Cup due to further ankle problems.
Ireland meet Scotland and Wales at the Aviva Stadium in the opening championship fixtures at the start of February.
And it could be the visit to Twickenham to take on beating World Cup finalists England before Sexton is able to return.
