Sean O'Brien underwent minor surgery on his hip and will miss the start of Ireland's Six Nations campaign.
Leinster flanker O'Brien has not played since the 22-17 European Champions Cup victory over Exeter Chiefs on December 16 due to tightness in his pelvis.
The British and Irish Lion struggled to overcome the issue and consequently went under the knife. He is not expected to be available until after Ireland get their Six Nations campaign underway against France on February 3.
It is another blow for Ireland, who will also be without Leinster centre Garry Ringrose for their opening matches as his injury problems continued with an ankle issue sustained at the weekend.
A Leinster statement read: "Sean O'Brien saw a specialist this week for the tightness in his hip as it hadn't responded to treatment as we would have liked.
"As a result he had a small procedure and the target is to have O'Brien back available during the Six Nations window."
Leinster could clinch their place in the European Champions Cup quarter-finals by defeating Glasgow Warriors at the RDS Arena on Sunday.