I like him but I don't think he should be playing every game, need to learn composure that will come with time but right now he's too rash.
How can you learn composure? Imo, Minutes and confidence! Just look at Vinicius, he had the worst composure of all TOP5 best leagues and now he is smashing it, Zidane goes, Ancelotti came and then the italian said Vinicius was his starter and he will play every match, well... nowadays, Vinicius is arguably the world's best winger with one of the best finishing in the world. All Vinicius needed was continuity and someone who had trust in his game.
Look, I'm not saying that this is the correct approach to use in every situation, because there is the equifinality concept, which says you can reach the more or less the same "end result" with different paths. Still, I think it's unfair to drop Martinelli off the team after this performance against City, if that's the case (ESR as a starter) I would expect a really bad and dull performance, not one where he missed a chance, but was arguably the most alive player on the pitch.