Arthur simply isn't that talented in the forward game. Lot of people can show Xavi control and pass stats in passing sideways/backwards. You need to do defense splitters or I'm not THAT impressed. I expect him to be a top 10 midfielder in the world at his best (if he ever has the discipline) or maybe top 5. Which is very good but nothing legendary.
It is more likely that he will be be on a verge of being a starter and a sub player for Barca than him turning into one of the best midfielders in the world:
1. horrible forward game
2. questionable defending
3. poor pace
4. horrible stamina
5. weight problems
6. poor professionalism
Even with a perfect professionalism and a perfect stamina, he would be a 30% of Xavi.
When you add all of these other issues, you get: it is what it is