He just isn't going to be able to pitch enough as a reliever for some team to give him Jeurys Familia money, let alone Corey Knebel money. As a starter he is guaranteed the former (having made $5.7 million this year) and could easily make the latter (or more with incentives).
Now, perhaps the Phillies could make him a creative offer that includes incentives for both games started and relief appearances. But if you do that, he'll want to be a starter. And if there's one team out of 29 that wants him as a starter, they'll have the best offer. It's not like with Brett Myers' time in the bullpen - similar situation as far as injuries, but he had to pitch for the same salary and for the Phillies regardless (as does Eflin now, but not next year).