Tomas O'Leary is facing up to six months out after undergoing surgery on his back.
The London Irish scrum-half is recovering after an operation to repair a "degenerative pre-existing injury" which has kept him out since 28 October.
Irish revealed the news on their website and estimated his recovery period at between four and six months.
That prognosis ends any chance O'Leary had of earning a place in Ireland's Six Nations squad and could rule out the 29-year-old until next season.
O'Leary has made just nine appearances for his new club since arriving from Munster in the summer.
A London Irish statement added: "Tomas originally suffered an injury to his lower back in the Aviva Premiership match with Northampton Saints on October 6th and aggravated it during the first half of the Aviva Premiership match with Harlequins on 28th October.
"Following further medical examination the decision was taken that surgery was necessary to solve this problem.
"Tomas underwent surgery on 4th December. Given the serious nature of the injury and subsequent surgery, Tomas is not expected to be available for selection until April 2013 at the earliest."