I would say we do have a pretty deep front office it's just that Fuld is currently on leave, and the implication of all that is that he and Mattingly will both have positions that are higher than GM, just Fuld on the biz side and Mattingly at PBO.
If the Nats wanted to give Mattingly that job two years before the Phillies can, he'd have to listen but he's only been a GM for half a year, so seems doubtful.
But Phillies have multiple people other teams should be interested in as a possible GM [if not PBO] - Velandia, Kilambi and even Ned Rice (though he seems be career assistant material, he's more the technical/contract guy).
Howie Kendrick would also be an obvious poach for them if he wants to move up.