The Leinster captain suffered the injury just before half-time at Stadio Flaminio, and hospital x-rays have confirmed the worst fears of the Ireland management.
He will be sidelined for between six and eight weeks - ruling him out of Ireland's remaining championship matches against France, Wales and England.
It is a desperate stroke of mis-fortune for the 32-year-old, who was recalled after a three-year absence following the retirement of Peter Clohessy and has managed to rekindle his international career.
He won his 20th cap against Italy, and his absence leaves a massive hole for coach Eddie O'Sullivan to fill.
Munster's Marcus Horan, who played the whole of the second half against the Italians, is set to fill in against the French in Dublin on March 8.
With veteran front-rower Paul Wallace still troubled by a shoulder injury, the selectors are likely to dip into the A team where the Ulster duo of Justin Fitzpatrick and the uncapped Simon Best could come into the reckoning.
Ireland are already without hooker Keith Wood with a long-term shoulder problem and went into yesterday's match without three first-choice backs in Girvan Dempsey, Ronan O'Gara and Shane Horgan.