That's the risk which comes from taking on players being pushed out. For every Di Maria, there is a Robben. Do you think Wesley Sneijder wanted to quit Real Madrid and joined Inter Milan? It didn't stop him from a treble at Inter with Mourinho. Similar to Eto: Being pushed out of Barcelona to make space for Ibrahimovic, he responded in kind with crazy workrate and effectiveness for Inter.
So it is not the same case for every player who is pushed out of their preferred club unwillingly. We need to look at the context. And De Jong's context is he would have his old manager, the one who elevated him to the highest point of form in his career, right here at Manchester United. And Ten Hag, who seems to value work ethics to a high degree, thinks greatly of De Jong too.