It has nothing to do with their ability to defend and everything to do with their ability to play football, again it's only my opinion so open to interpretation. I wouldn't criticise Aguero for a defensive mistake if he lost possession and the opposition broke away and scored for example, as that's what he's asked to do. Same with Otamendi and Zinchenko in those situations, they're asked to play, even when the situation possibly warrants them to kick it 80 yards into row z. We take risks in possession, I don't see those as defensive mistakes but each to their own.