Why wouldn't the Tigers consider dealing Castellanos? 5 of their top 10 prospects are outfielders. The Tigers felt highly enough about Avisail Garcia to call him up before Castellanos, I'd assume that he isn't untouchable. Besides, the Cubs and Tigers engaged in trade talks last year centered around Garza and Castellanos, and the Cubs were the ones who balked. I'd assume that wouldn't be the case this year. Castellanos is ready to play in the majors.
Also, why are you assuming that Garza would be a rental? In my post, I named almost a half-dozen players that the Tigers have given sizable extensions to. On top of that, the Tigers have shown in the past they're willing to build on their strengths (starting pitching, power hitting) and ignore weaknesses (overall team defense, closer). I'd be fairly confident that the Tigers would be able to agree with Garza on an extension. Plus, they have another year of arbitration with Max Scherzer, so his sizable raise won't come until 2015. Plus, the Tigers could view Garza as a replacement to Scherzer going forward, because I'd assume he'll be getting paid the higher salary between the two. They could end up shopping Scherzer this offseason or during next year in order to recoop some of the prospects lost in any trade involving landing another arm. Anyone should assume that a trade involving Soriano would have the Cubs paying a substantial portion of his salary. That isn't even a concern on my end. My biggest one, for the Tigers, would be them giving up an additional two pitching prospects, given that they don't have tons of high ceiling guys. Assuming how much the Cubs are willing to pay on Soriano's end, that could push them over the edge.