Dublin - Irish No8 Jamie Heaslip has three fractured vertebrae and will miss at least four weeks including the potential Grand Slam decider between Six Nations champions Ireland and England in a fortnight it was announced by the IRFU on Monday.
The 31-year-old British and Irish Lions star suffered the injury after being kneed in the back by France lock Pascal Pape in Ireland's 18-11 victory over the French on Saturday.
"Scans have revealed fractures of the transverse process of three vertebrae in Jamie's back," read a statement from the Irish Rugby Football Union.
"Although this injury causes a good deal of discomfort it does not impact on the structural integrity of the spine, and once healed should pose no long term issues.
"Typically this injury is treated akin to a soft tissue injury; according to symptoms.
"Jamie is already feeling more comfortable and it is hoped that he will be available to play again in approximately four weeks."
Pape, who was yellow-carded for the offence, is to appear at a disciplinary hearing later this week after being cited by the Six Nations citing officer at the match Italian Stefano Marrama earlier on Monday.
The Stade Francais lock could be suspended for up to 52 weeks, although the usual punishment is an eight-week ban.
Pape's presence against Wales in France's next Six Nations match on February 28 is now very unlikely.