Imo a lot of times he's made the wrong subs and there never seem to be any tactical subs it's usually just a striker for a striker, winger for a winger replacement. Luiz playing ahead of Conor against Man City and Liverpool is the obvious one.
Dean shouldn't be afraid to tweak his system during certain moments in a game to get a result.
We all thought he would use him in a holding role together with Nakamba to make us harder to break down, but he used him in the same exact role where Conor operates.
Dean is really basic in his in game management.