IMO this was theo's biggest error. But he has time to fill up the team with some pitchers coming up in the draft and like you said look at trading someone like a Castro for two TOR type guys in A ball or something along those lines to help rebuild. The Cubs have a position of strength at SS which is why I used it as an example.
Also with Vogelbach, Baez, Alcantara and Almora, I could easily see them as the most expendable, Castro on the parent club because like you said, represents a position of surplus, and Bryant, Russell, Soler, and Schwarber as probably the safest to stay as far as the youth.
Maybe they can get by with a Latos, Porcello or something if they become available. I didn't really see the Royals doing what they did with Shields, Guthrie, Vargas, and Ventura in the rotation, so maybe the Cubs are not that far off in the pitching department. An ACE would be nice, but I have seen teams win it all with less.