I can't comment on Sibbald but McKay has undoubtedly been one of Rangers' best players this season so it's a well-deserved call-up to the squad. He's always had a brilliant touch and he now seems to be improving his decision-making, he'll make a short pass to retain possession rather than swing a ball into an empty penalty box, so that's a maturity young wingers often don't have.
As far as I can tell the U21 squad is similar to the first team squad ie not much quality at CB or CF but plenty of wee attacking midfielders/wingers!
