Well, if the Sox want Castillo, they are going to have to give the Reds something good. If the goal is to hold on to Casas, Mayer, Yorke, and Houck, it gets much more difficult. Jarren Duran only gets you so much.
Even taking back a questionable contract like Suarez (and then trying to figure out where he plays) doesn't really help the situation much. Taking back Moustakas might, but at some point, Cincinnati is probably going to ralize it is unwise to dump their worst contract in a package with their best trade bait...