Important for utility guys to be able to fill in adequately at the key positions--center field and shortstop. Rocco prefers Farmer over Castro at short and has been somewhat reluctant to go with Willi in center field. According to BBRef, after more than an extremely SSS, Castro is a neutral in center. While that doesn't sound great, being average where the best outfielders in the world are deployed is pretty good.
When both players were on the active roster, Rocco used Castro at short. Again, that is telling to me in any competition for a utility spot on the Twins going forward.
I would agree with those of you that are saying Willi is good, not elite, in the field. His best position appears to be left field, although he's played precious little second for the Twins, because they had other less-versatile options there.