Herrera - 1.8m
Fellaini - 1.92m with impeccable chest control
Fellaini is the only player in the team who can do a good job when we use the long-ball/early crossing tactics whereas Mata/Rooney/Di Maria can all do a similar job the Herrera. That's the only reason I can think of, that Fellaini adds another dimension to our attack.
That's the trouble though - I want to see a team play the kind of football Herrera is good at, not the kind that needs a long ball to Fellaini. It bothers me that we'd buy a player of Ander's type if we didn't want one.
I do wonder if Herrera is struggling with the court case though. Even if you're convinced you're innocent, legal proceedings can be a huge emotional strain. That can have all kinds of negative knock-on impacts in terms of frustration, loss of concentration and depression. Of course a fractured rib and a manager who doesn't trust/need you might put the top hat on that.