Howard was a perfect storm of age, injury and one-dimensional skills (as well as the shift). I am not necessarily a Castellanos believer, especially not relative to his salary and term, but I think if he's not cooked his bounce back will be better.
But you're right, they don't really need him. Once Harper is back especially he's just one of several DHs. If he is cooked it doesn't really matter whether he is tradeable or not. They will have to move on, or platoon him with Schwarber at DH for one year while not ever playing him in the field or against RHP. And they can find some cheaper/creative options for another OF besides Rojas, or at 3B if Bohm also ends up as a DH/1B once Rhys is gone.