Fred’s a curious one, he is excellent with the NT team and not great with United (who is?!). Yea, guess its the fact that he has too much to do at United and with the seleção hes passing it to world class players
For Brazil he plays next to Casemiro or Fabinho. At United they expect him to be Casemiro
Also for Brazil along with Casemiro he usually has Paqueta, who's versatile enough to do anything in midfield, next to him. And most importantly what I see for Brazil is that Neymar is always available to Fred for a quick 20 yard vertical pass. Any time Fred picks up the ball he has Neymar as an outlet.
Now when you compare that to United (where he is in Casemiro position), has McTominay next to him (who is the opposite of Paqueta, basically can't do anything) and Bruno who is always out of position (living out his dream of playing as a striker).
And then instead of Neymar he has Ronaldo who can't hold up the ball or enough pace to run in behind defenders. Or Rashford who is permanently offside