I think we need to cut Zinchenko a little slack, don't you? I understand we're gunning to be the best side in Europe and that we can't have deadwood holding us back, etc. etc., but he's a kid being asked to play out of position because our other two left-backs are so injury prone. He's going to look poor playing as a defender on a long-term basis because he isn't one. He played as a winger/#10 for his entire youth career until Mendy and Delph were both on the treatment table. I understand Delph wasn't a left-back either but some players are just naturally suited to certain positions, even if they've never played there before - Zinchenko just happens to not be one of them, that's not really his fault.
I wanted to keep Zinchenko over the summer and was sad when I thought he'd be heading to Napoli, but I think he needs to leave in the summer. Not because I don't want to keep him - I think he's a great third option to have, and I think quite a lot of clubs in this league would prefer to have him as defensive back-up than some of the options they currently have. The fact that Newcastle have been alternating between Ciaran Clark and Javier Manquillo at left-back should explain how lucky we are to have a deputy like Zinchenko. But he needs to look at his options: he's behind Mendy and Delph at left-back, and behind both Silvas, De Bruyne, Sterling, Gundogan and Foden for that #10 spot.
He needs to weigh up his options in the summer, and I think when he does we'll see him leave.