I like the player but unconvinced he would be a good fit. Traditionally speaking, he plays really as a left winger or in the Iniesta role but both spots are occupied. Even as an heir to Iniesta, I don't believe he possesses the sort of game to play that role similarly - he's more of an attacker than a player who controls tempo and directs play.
Still, under Valverde not sure how he might best use a player like Coutinho when there is already a number of decisions to be made concerning how to use Rafinha, Denis, Iniesta, Roberto, Sergio, Gomes, Rakitic, Arda, Mascherano, Samper & Aleñá
Before season stat, I expect there to be a lot of changes in midfield. By comparison, Pep used to have a squad with 5 midfielders + 1-2 youth mids