I agree that his IQ is the main problem but some of you should realize that he is a WINGER! In Valverde's system players which suffer the most are wingers. We basically don't have them. Look at yesterdays game. Our wingers were Coutinho and Rafinha, none of those two is a classical winger. In the end Sergi Roberto played there! We aren't playing classical 4-3-3 formation and that would be a problem for every winger not just Dembele.
Neither is Dembele tbh.
Classical wingers in the old school sense were the ones who play beside the sidelines and aim for cross,pass etc. sort like Deulofeu when he plays on RW.
Modern days wingers are usually more scoring oriented, the way Messi and Cr7 have played in their prime, or even Hazard or Neymar.
Dembele doesn't do either, he has been an AM/Winger hybrid. It was supposed to be one of his strength during BVB days, some even argued AM is his best position because of that (which I think was wrong argument)
A player like him should actually fit nicely in current system and yet it is the opposite.
You can make the coach argument, although I still think better coaches won't tolerate him as much as people thinking but we will never know until he has a new coach.