If we're betting on Arthur it's because we/the club believes he has the potential to become a world class midfielder. Do we believe that is the ceiling for Mina, and are we gonna bet on that? If it was the money alone, <10m for Mina is nothing and he'd be absolutely worth it. But the non EU spot directly pins him against other non-EU transfers, I have much higher hopes for the potential/ceiling of Arthur than Mina personally and for what he could add to this team.
Even with Coutinho, we'd be in a limbo for our other midfield spot. Aging Iniesta and Rakitic are covering for now, but how long can that go on? Our best bet for a rising high quality replacement at that spot is Aleña and he might be years from reaching that level and he'd be best growing as a player in rotation. Arthur could develop into a proper starter level player by like mid 2019 if not earlier which goes along with the timeline we need to follow to phase Iniesta out. Also if he ends up that good we'll have true long term depth in the MF with no lack of quality at all, which would be one less major thing to worry about going forward (the biggest one being post-Messi).
All of this is irrelevant tho if we can solve the non-EU problem. I've read some people say it's Suarez taking up a spot, some say it's Coutinho? If it is Coutinho and he can get an EU passport soon enough, this whole issue is null. We wouldn't even "need" to buy Arthur short term with Coutinho here so we can hold off till the passport issue is settled.
