I don’t think this is an issue until Diaz is ready. They can explore what they have in Mountcastle. No need to make a decision now. As others have said Mullins is more of a bench player. I would not deal Santander myself. Switch hitters with his skill set are not easy to find. Do I think he is a lock to be good?, no but he has a chance. Mancini doesn’t even play into this, he was a 1B who played OF due to need.
A creative manager using DH can give all these guys at bats next year. Nunez could be moved as well it needed.