I think it's because management doesn't trust him to string together two solid years. They might feel that his ability lies solely in his delivery, and that he'll be repeatedly hammered once hitters catch onto it.
What a relief this game was. The only thing that would have made it perfect is if Perez could have held on for a little longer. As it is, he pitched long enough to make long relief tomorrow a possibility if necessary, which is what's important.