Without any exaggeration I will tell you that his performance away at Villa before he got injured was worse than ANYTHING he has served up since, and probably in the top 5 worst individual performances i've ever seen from an Everton player. And I saw each and every one of Cuco Martina's games for us.
I missed that one, but sounds like I didn't miss a lot.
I think his issue, is a bit like Davies's issue. He takes lots of risks in possession. Before people got used to him, he got away with it, but once they clocked him, he just became a bit of a liability. He will still have the odd game where he looks great (Brentford at home) but in England, playing against big powerful players he just gets the ball taken off him and he loses confidence.
Davies is similar really. If you watched video's of him, you would say go and press him, he will make a mistake.