Ireland flanker Sean O'Brien has been ruled out for "five or six months" after the shoulder he had surgery on last season became infected.
Leinster coach Matt O'Connor confirmed the news on Thursday which he described as a "nightmare" for the Lions flanker.
O'Brien, 27, will miss the pool stages of the European Champions Cup and Ireland's autumn games and could be ruled out of the entire Six Nations.
He suffered his dislocated shoulder in a game against Ulster last December.
That injury caused him to miss nearly all the remainder of last season, including Ireland's Six Nations triumph.
|Sean O'Brien factfile|
|1987: Born 14 February in Carlow|
|2007: Part of Ireland's Under-20 Grand Slam winning squad|
|2008: Leinster debut against Cardiff Blues|
|2009: Helps Leinster win Heineken Cup and earns first Ireland cap against Fiji|
|2010: First Six Nations appearance against Italy|
|2011: European player of the year after helping Leinster win the Heineken Cup|
|2013: Plays two Lions Tests against Australia but suffers dislocated shoulder later in year|
|2014: Ruled out for rest of season after shoulder setback in October|
O'Brien did start in Leinster's opening Pro12 game against Glasgow last month but is now facing another long spell on the sidelines.
"If all things go to plan he'll probably have shoulder reconstruction surgery tomorrow," added O'Connor on Thursday.
"There was a little bit of a complication with his previous surgery due to the infection and they are going to have to go in there and redo that.
"He's extremely frustrated but the positive thing is that the ankle procedure he had a couple of weeks ago was a lot more minor than we thought."