He is 21, & the biggest criticism he is getting, is that he 'may' not be good enough, as a midfield player, a position he hasn't been allowed to play, for one single senior game, at his club. He was instead, shoehorned into an alien position, in which he did his best, to help us out.
And he is supposed to become top class as a midfielder, instantly, wheras if Mahrez starts the season poorly, most people will (rightly) say "give him time to settle in ffs". Yet he is a p.o.t.y. who has played in his favourite position, for years.
So Zinchenko (& Foden, Diaz etc) all have to become as good as Silva, Mahrez & Co, pretty much straight away without having the opportunity to actually play, for years, like Silva, Mahrez etc had.
Silva, Mahrez, Fernandinho etc, would not be getting a regular game, in this team, at 21, if they had been here & never played the position at this club, previously.
We are basically telling Zinchenko he has to be better than they would be under the same circumstances, or sold.