A keeper? You'd better hope he lasts until 54.
Look, prospects like this guy come along once in a lifetime. The Mariners are even taking steps to ensure that his career is a long and successful one.
Word is that they're going to hold him to 190 innings this year.
Actually, i won't bother to write anything else, because here's a good article on that whole situation:
Mariners playing it safe with Hernandez