It seems similar to how Verratti can't just leave PSG.
I don't think we can sign Coutinho regardless of player's willingness to leave.
Clearly you haven't been close to transfer windows in the past years.
1.A team makes an attempt to sign a player
2.Current clubs declares him untranslatable
3.Player quietly leaves a transfer request
4.Current clubs sets a fixed price for the player and the other club pays after some short negotiations.
5.Presentation a few days later.
It's exactly the classic situation that has happened every year in the past 20 years.