Ireland's O'Brien in doubt for Six Nations
Rugby player Sean O'Brien passes the ball during a captain's run in Sydney on July 5, 2013
O'Brien, who made his British and Irish Lions debut in the second Test against Australia in June, injured himself during Saturday's inter-provincial clash against Ulster.
The 26-year-old was forced off after 53 minutes in the tie which Leinster won.
"Sean is to have surgery this week to repair a dislocated shoulder, which will unfortunately keep him on the sidelines for several months," the club said on their website.
Leinster forwards coach Jono Gibbes said, according to the Irish Rugby Football Union's website: "The time frame to return to play hasn't been set in stone but it is significant enough for surgical intervention which means six to eight weeks minimum.
"It looks like that (he will miss the entire Six Nations), but they are not going to put an absolute line in the sand for him. They are just going to see how it goes."
Ireland open their Six Nations campaign against Scotland at the Aviva Stadium on February 2.