Fred is a good player who, like Fernandinho, impressed with Shakhtar and deserved a move to the PL.
However, unlike Fernandinho, he doesn't really have an out and out major role in CM and needs the right blend around him to look at his best (similar to van de Beek).
He offers good energy and work rate, but isn't a good enough defensive midfielder to play that holding role on his own. And he is OK on the ball and can make some attacking contributions, but doesn't offer much end product in terms of direct goals or assists. He's a very useful #8 if the blend of the other midfielders complement his strengths and weaknesses - again, similar to van de Beek.
He's a player that we can definitely do with in the squad but, ideally, as a regular starter, we'd have a better player that's Man United standard in his own right regardless of others around him and who offers more of a major role in either the defensive or attacking part of the game.
But, again, like someone like Daniel James, a very useful player to have in the squad and to pick for certain types of games.