I don't think Mo's successor will come from within the organization. It'll probably be someone we acquire via trade or FA signing. Eric Gagne is a name to look at because he's going to be bound with short term, probably 1 and 2 yr deals for a while and he'd be someone who would be very accessable when Mo retires. The other guys in the system are good but I think the front office will look at someone who is proven in big game situations and who can get the job done instead of relying on a young kid. There are plenty of closers who will be available when Mo retires in probably 5 years or so so we really don't have to worry about this for about another 2-3 years.
The Yanks will be evaluating Humberto Sanchez in spring training. He'll be auditioning as a starter, not a closer. They might decide he's better suited to closing, but I'm sure if he can't win a spot as a starter he'll make the 25 man as a long reliever/spot starter.