Its a tough call. The Twins could desperately use another effective starter and if Duffey has the potential to fill that role then you want to make sure he is given every opportunity to prove he can't. If I'm choosing between Berrios and Duffey to replace Gibson I'm going with Berrios though.
I agree with the Trevor May comparisons. I doubt the Twins would force Duffey to continue starting in the same way they've kept May relegated to the bullpen but if he does have a good thing going I'm ok with him staying in the pen when there are better options to fill the vacant starter role.
I think the real dilemma is if/when Hughes gets bumped. At that point it should be Duffey stepping into the rotation.