Arthur's signing could see one of Barcelona's non-EU players forced out, and that means it's most likely Yerry Mina, given Paulinho and Philippe Coutinho are essential members of the squad.
Barcelona have asked for a Portuguese passport for Coutinho, because his wife is from there so he can get dual nationality, but they know that it will not arrive for January 2019 as planned. Sources close to the player know that, despite doing everything they can to cut the time.
The idea was to sign Arthur in January 2019, as Gremio wanted too, with the club aiming to get Coutinho's passport by then. But with Arthur now set to arrive in the summer, Mina may have to leave. The club know he's a work in progress but the reality is he's not had a lot of minutes.
Various teams will want him on loan, some lookign to buy too. Barcelona like him a lot and would look for a loan so he can acclimatise to European football. Now Barcelona have to look for another centre-back, if Mina goes.