Seeing a number of people say that one of the reasons Liverpool ultimately didn't go with him was that he's basically 100% set on playing a back 3 all the time and the rest of the Liverpool footballing setup wasn't sold on that being the best formation either for their current squad or for recruitment moving forward.
If true and he's is insistent on implementing a back 3 at United, it could get worse before it gets better. If they're healthy I think they have the CBs to play a decent back 3 (Yoro- De Ligt - Martinez seems pretty balanced) but they're severely lacking a real wing back. I think Shaw could do great in that role since he's a great player overall and very versatile, but he can't remotely be counted on to be healthy.