O'Brien, who was part of the British and Irish Lions' team that beat Australia earlier this year, will have surgery on his shoulder after picking up the injury during a club game against Irish rivals Ulster at the weekend.
"Sean is to have surgery this week to repair a dislocated shoulder, which will unfortunately keep him on the sidelines for several months," Leinster said in a statement.
Asked if he would miss the whole of the Six Nations, which runs from February 1 to March 15, Leinster forwards coach Jono Gibbes told journalists "It looks like that" but said no final decision had been made.
(Reporting by Conor Humphries, editing by Pritha Sarkar)
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