Manchester United midfielder Kobbie Mainoo impressed after coming off the bench at half-time in England’s draw with Slovenia. England should not be in a position where they are depending on a teenage midfielder for a boost, but neither really should Manchester United. And sometimes, the player is simply too good to leave out. Erik ten Hag recognised this for United, selecting Kobbie Mainoo is nearly every possible game he could. England boss Gareth Southgate has shown a bit of restraint so far, refusing to select Mainoo as a starter in any of England’s group games. Mainoo came off the bench ag…