Jamie Heaslip is unlikely to feature again in Ireland's quest to retain the RBS Six Nations title.
The 31-year-old No 8 was forced out of Ireland's 18-11 victory over France on Saturday after receiving a knee to the back from Pascal Pape, who has since been cited for the challenge.
Ireland bosses confirmed Heaslip suffered "fractures of the transverse process of three vertebrae" in his back, leaving only a slim chance he could return for the final match against Scotland on March 21.
Head coach Joe Schmidt admitted Heaslip was "very uncomfortable" straight after Saturday's second-round victory, while Pape now faces a possible ban for his crude challenge.
"Scans have revealed fractures of the transverse process of three vertebrae in Jamie's back," read an Ireland statement.
"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."
The 70-cap loose forward (right) binds together the industry and grit of Peter O'Mahony and Sean O'Brien in Ireland's first-choice back row, and his absence is a clear loss to boss Schmidt's side.
Ireland host England in Dublin on March 1 in a match already billed as the Six Nations title decider, with the two teams the last remaining unbeaten sides.
Heaslip's injury opens the door for Leinster back-rower Jordi Murphy to step up for that all-important England clash.