Love Coutinho and he was absolutely needed. However, he surely is not fitting into a midfield position as of now. And he seems pretty poor defensively. So I'm not certain about the long-term viability of him playing in a 4-3-3 until he improves in that aspect. Have absolutely no clue how he would fit into a successful system with Griezmann, Messi and Dembele (and maybe Suarez).
He provides an injection of quality on the ball that Barca were sorely missing though. And that in itself is enough to make him a starter in my eyes.