From Joel Sherman of the Post:

If Mariano Rivera's health falters and he is unable to close, the likelihood is Joba Chamberlain would be switched into the role, multiple Yankees officials told The Post.

... if Rivera were to miss a sustained period, three club officials verified that Chamberlain almost certainly would replace him. The officials admitted the subject has been discussed at the highest levels and conceded Chamberlain has the best combination of temperament and stuff - by far - in the organization to handle both replacing the legendary Rivera and coping with the stress of being the Yankees' closer.

Thankfully, Mo's right on track, so hopefully this is a bridge we will not have to cross.