I get where you're coming from, but I wouldn't entirely agree. Obviously Dybala will look better in a team with lots of movement beyond him, but he does have two or three special qualities that make him a top player in any team. His close control, ability to beat players in tight areas, and ability to score from distance mean he is one of those few players who can genuinely create something out of nothing. He probably won't be a consistent "game changer" as you put it, but he will be able to produce moments of magic that will win games for us, which is not something you could say of any other player we have bar maybe Pogba.In a good functioning team, with lots of movement around him and in a team who can also get him into positions closer to the goal where he can hurt the opposition he would be brilliant.
In short, if people expect him to be a game changer on his own I think they would be a bit dissapointed. He is not that Hazard type of player while being in Chelsea. On the other hand, in the right system he has potential to offer even more then Hazard who is too individualistic at times imo.
So, a lot will depend (like most of the times) on our manager. Am all for it because he is techincally brilliant and step towards more progressive type of football we would like to see in United. Get him. In fact, get more players that have his technical ability...