Frankie De Jong not happy nowadays with Barcelona after several match not as regular player, he has dedicasion leaving Barcelona next year, but Manchester United is not his next challenge because not qualify to champion league make Frankie De Jong not interested moving to Manchester United. Very hard for United getting midfielder replace position left by Pogba and Matic because not participants on champion league make many player dislike moving to Manchester United.
I don't know what's happening in Barcelona, Frankie De Jong should be a regular player there since no other players are better than him. If he often sits on the bench, it is very understandable if he wants to move to another club in the next transfer window. But I don't think moving to Man United is a good idea, what Frankie De Jong expects from this club? The current situation in Man United is very bad, there is no hope to win any title with the current performance of Man United. So, it is better to move to another club if any other offers. I'm sure there will be many offers from top clubs if Frankie De Jong will be put on the list in the next transfer window.
Depending with Xavi Hernandez not really interested with Frankie De Jong and prefer give chance for young player Gavi on midfielder area than De Jong, true about how quality and level has by Frankie De Jong but after Koeman leaving Barcelona not give good position place for De Jong. Manchester United try give guarantee as regular player for Frankie De Jong and have good reunion with Erik Tan Hag but not interested because Manchester United not play on champion league next season.