Talk on the Hunt for Reds October podcast (featuring Doug Gray, btw) mentioned that Suarez and wife have a baby on the way, and he might opt out for the season. If that's the case, the Reds might just have lucked into having both Moustakas and Castellanos on the roster. Either could play 3rd -- Moose would be my pick as regular, Cast would be used in a pinch. Having Castellanos there regularly, though, plus the DH, would open things up in the outfield as well.
Anyway, this is just musing at the moment until there's official word on Suarez.