I could be wrong, but I don’t think Castellanos was even that bad defensively in Wrigley. GRF is not like Comerica. I think the concerns over Castellanos’ defense are slightly overblown.
Maybe the plan is:
2020: have Mazara play RF with Castellanos at DH. If Mazara does well, they’ll tender him and then trade him, allowing Castellanos to move back to RF for 2021
2021: Castellanos in RF, Abreu at DH, Vaughn at 1B
They might purposely use him less next year to avoid it becoming guaranteed. It might actually be beneficial for both sides because using him less would probably keep him healthier.