Manchester United may have made a mistake when they let Jack Butland go last summer with the former England international showing the club what they are missing at Rangers. That is the view of Ibrox icon John Brown, backing up the point made by another blue-clad legend in Ally McCoist. McCoist, after yet another virtuosoJack Butland display, labelled the experienced shot-stopper one of Rangers’ ‘best signing in years’. Brown goes one step further. He believes that Butland would walk into the starting XI of most teams south of the border, and he includes Manchester United in that. Jack Butland …