Arthur is still adapting to a team as fluid and offensive as Barcelona. The level of play at Gremio was a lot lower, imagine when he plays next to guys as dynamic as Messi/Coutinho.
The fact that he's so well adjusted already is impressive, it looks like he's making strides every single game and he has yet to train with the best players. So many of our best attacks today have come from buildup directed largely by Arthur.