A second such break for the Corkman - following his sidelining in December 2003 when he fractured his leg against Coventry - Healy, who only returned to club training last week, has now suffered a double fracture in exactly the same place as his first break.
Healy was out for nine months last time, and this subsequent injury will rule him out of much of, if not all of the remainder of Ireland's 2006 World Cup qualifying campaign.
The only saving grace for Healy is that he is just 24, and has time on his side.
Black Cats boss Mick McCarthy told the club's official website: "I'm absolutely as sick as I could be for Colin and of course we'll do whatever it takes to help him through this."
Healy completed a free-transfer move to Sunderland in August 2003. The former Celtic midfielder had a spell on loan to Coventry during his five-year spell at Celtic Park.