Yeah he played LW in his career but you gotta look at the context. You can't ignore complementarity with other attacking partners.
Who were his partners in attack ? Playing LW at Liverpool with super mobile players such as Firmino and Mané gave Coutinho the wide creator role. A lot of AM can perform that way. Iniesta played a shit ton of matches as LW too. Pastore played on the wing too, James too, Ozil, Eriksen, Mata, etc. That doesn't make them natural wingers.
Problem is that Coutinho can't play LW in any team, or in any system, or with any partners. At Barça, the (wide) creator is Messi. The other guy is slow and old Suarez. There's absolutely zero point in adding Coutinho as the third attacker. Coutinho is as slow as Suarez and wants the ball at his feet in order to deliver the final pass, like Messi. I already talked about it before. Coutinho as LW could work if you partner him with Dembélé on the RW and Malcom as the number 9. Because these 2 fast and energetic players will do the runs and Coutinho will set them up.
When you play Coutinho in attack with Messi, Coutinho is the one who will have to do the runs and he simply sucks at that, he's slow and unexplosive. Add Suarez as striker and we have a soft and limited attack.
When Coutinho plays in the midfield (AM or CM) he delivers a shit load of key passes, adds much more creativity to the team. But in midfield too he has clear weaknesses which are his lack of discipline and lack of defensive contribution.
Now you have to ask yourself: what's easier to learn for Coutinho, discipline and stamina or pace and explosion at age 26 ?
To me the answer is clear as water and he should play CM/AM.
I knew we'd have this problem before he came here. And I knew that the incompetent idiots who run this club (board + Valverde) would have no idea how to solve it. That's why I was against this transfer. Valverde playing him LW the 10 last minutes of our matches since november is absolutely pathetic. He's completely killing the player.