You know the drill. Michael Oliver, the referee in the Manchester United-Arsenal FA Cup quarter-final, cannot be praised for a brilliant performance with cards and whistle if he has made so much as a single error in the past. He sends off Angel di Maria, correctly, and the howl goes up: “Yeah, but in the Capital One Cup third-round two years ago he let so and so stay on the pitch after a foul in the box, so all credit is withheld.”