What happened to this guy? He actually started out well, then faded away badly.
I still believe it is the same as Hleb: something personal happened that fucked him over.
I remember back in the day folks talked about how Barca destroyed Hleb, and I insisted there is no way this is the case. You don't lose basics because of one year in a bad fit of a club.
Hleb later admitted it, it was all him. He was in a rough patch in his personal life and it affected him. How he basically rejected to learn Spanish or follow coach instructions.
IMHO, there is no way Coutinho dropped the way he did for just football reasons. Something happened in the background, that we may never know off.