Is there anyway that Roark gets bounced from the rotation before Thornton does?
Roark seems to me to be a pitcher that gets his value from eating innings, consistently, over the course of 30+ starts. Not necessarily someone who is exactly eye popping in a 60 game sprint.
Thornton as a rookie showed flashes of being a pretty good pitcher from what I recall, and the two are very comparable in batted ball profiles, strikeout rates, and walk rates, this isn't even factoring in what division Thornton played in and that it was his rookie season. I know they paid Roark, and he'd probably little value out of the pen, but I Think it would still be worth exploring keeping Thornton in over Roark for this 60 game season.