not a chance in hell you should make that move. cantu is your 2b right now and he is still better than castillo imo. getting castillo only gives you depth. Sanchez may be overrated (who really knows?) but with a guy rated that high, you dont wanna deal him unless you are getting something you can for sure use. what happens next year if cantu comes into the season healthy rather than hurt and goes back to producing like he did? then castillo moves to your bench and wont see the light of day. and you dealt a big time prospect to get a back up. IF, and i mean IF, you really feel that castillo is better than castillo, then i guess you help yourself now as opposed to later. but even if you see castillo as better it cant be better by much, so you dont improve much.
keep in mind that there is talk that sanchez has an outside shot at a rotation spot this upcoming season in Detroit. Its likely that he wont be in the rotation next season barring an injury. but it is likely that he gets a role in the bullpen. so depending on your league stat categories, sanchez may have a great value this upcomign season even if he doesnt end up in the rotation.
i would say dont do the deal because even though you get a "sure thing" in castillo who is already in the pros...he is a player who either had a fluke or break out year, and im leaning toward fluke personally. nt worth a possible fluke player to lose a guy like sanchez