How is it mind boggling? He's an uncapped 24 year old with a modest performance record. Admittedly Spain have ridiculous depths of riches in midfield, but by comparison Fellaini has 6 years of Premiership experience (this is relevant as we play in the Premier League) and 42 caps to his name. He's been the/one of the best midfielder(s) outside the top 6 for a while. Maybe one doesn't quite agree with the valuations but it's hardly mind boggling. As far as I'm aware Herrera has hardly ever set the world alight. He's just way overpriced because he's Basque.
What does this matter? How many caps would have Fellaini and Cleverley combined if they were Spanish? A grand total of 0.
As I said yesterday, Illarramendi has 0 caps for Spain, Thiago Alcantara has 4, Isco has 0. All top quality players. In a team that has Alonso, Busquets, Xavi, Iniesta, Silva, Mata, Cazorla, Fabregas, Martinez as senior players and has a very conservative manager, it is to be expected that some quality young players won't play. On the contrary, the team Fellaini has 42 caps hasn't qualified for any big tournament since WC 2002 while this Spanish team has won the three latest big tournaments. The moment you try to put in argument that Herrera has 0 caps while Fellaini has 42 is the moment you accept talking nonsense.
I agree with you that Fellaini is one of the best midfielders outside of top 6. He probably is even the best midfielder there. The problem is that outside of top 6 is his level. People had the same argument with Young, he's one of the best wingers outside of top 6 so he'll shine here. Well, he didn't. I sincerely hope that I am wrong, but just I can't ever see how Fellaini would be more than a squad player here (in case we really think to improve the midfield).