It’s never a good sign when a teenager looks like your most mature player. But that is the nature of things at Manchester United at the moment. And while Rio Ferdinand can hardly be more glowing about his praise for a player who only made his senior debut in the Premier League just before Christmas, he does have some rather nagging concerns about what sort of an effect Erik ten Hag’s system – the Red Devils playing something more akin to basketball these days while leaving wide open spaces for opposition forwards to gallop into – will have on Kobbie Mainoo’s development. It was a very familiar…