Castillo may never be back, as Dombrowski is reportedly not a fan, calling him a "fourth outfielder at best" during his showcase. But right now, the guy has had like 300ABs spread out over 2 years, and played through injuries for a good chunk of that.
His contract is a stumbling block, although not going over the LT limit this year removes many of those issues. He does play the outfield with reckless abandon, resulting in far too many injuries. And unfortunately for him, that style works.
And while Dombrowski initial assessment was a disparaging one, it is worth noting that the role DD pigeon-holed Castillo into is the very role the Sox might use him for....