I would think twice too.
As much as I think he should feel he owes the team, he really doesn't. The guy has never been on a winning team, and this next contract he signs could carry him towards the back and of his career. Does he want to take a chance that he'll never play for a winner?
If I'm the Dbacks, I offer Sexson a one year deal around 8 million or so. I would be hesitant to give him a big long term contract, seeing as we don't know how long it will take him to get back to form, if he will, and if this injury will happen again.
If a team like Seattle offers a large, long-term deal then I say we cut our losses now, say "tough luck for us", and move on.