He's a quality player but a victim of the changes.
Arthur becoming a starter fixed a lot of problems the Midfield had in terms of ball retention and possession and being able to control the game but also it shoved Coutinho into a spot (LW) that you won't get the best out of.
I think Klopp got the best out of him at Liverpool where he played as an LCAM in the middle of a 4-3-3. The difference is that team had a lot of movement which benefited Coutinho a lot. Over here that is quite a problem.
Seems like Valverde would rather give a Barca B player/Malcom a chance than to ever play Cou in such a role ever again if it means Rakitic has to sit.
Coutinho played LW for 2 seasons under Klopp, and only half season as AM.
I can't say it is pushing him on the left is the problem considering his history as a player, on paper that seem like perfect position for him, and he played well there last year with us too. He didn't play AM at all with us IIRC, either LM,LW or RM.
Also having stamina problem seems weird after his effort in the WC.
All that being said, something is wrong with Coutinho playing lately, he pad his stats but plays badly. And this is the one case I would blame the coach the most tbh. EV wanted him, got him mid-season and entegrated him gradually well. No reason that you don't have a clue how to maximize him.