Manaea has one season left before free agency. Montas has two seasons left before free agency. Oakland is going to want and get a boatload for both because that's what they do. It might not appear to be a boatload, but they will acquire players that will turn out to be very good ball players that will be traded away for more prospects before they hit free agency. Wash, rinse, repeat.
Castillo has two more seasons before free agency, but I'd feel more comfortable dealing with Cincinnati because, aside from Simon for Suarez, they aren't the kind of trade machine that Oakland is. Its a stupide rationale, I fully admit it. But Cincinnati has already made questionable deals this offseason, so it seems like that's the table to play poker at.
I've not been much of a fan of Pineda, but perhaps that's the Tigers' best available move. It'd be just money (which, of course, the amount and length of contract makes it relative). He's in awful shape and has had injury issues in his past, and I'm not sure I want to trust him with a pile of cash and get only 120 IP/season out of him. Maybe they can get him on a short term incentive laden deal? There's talent there, but can the talent take the mound every 5th day?
The farm system has improved, but I'm not sure that it is all that deep. And the major league team has improved, but I'm not sure its good enough to be dumping a bunch of prospects for major league rentals.