They can't, If I understand it, tell the referee to review for a yellow card, but they can tell the referee to review for a red card. Now, exactly what happens when the referee is asked to review something like this for a red card, but he doesn't think it warrants a red card, I have no idea. Can the referee change his decision based on the review. Or does he have to give a yellow to Casemiro, but leave United with the free kick?
What should have happened here imho, is ask the referee review the decision on the pretence of a red card, but then get the decision right even if it is a yellow.
The whole thing is a shambles. None of it makes any sense.
VAR cant correct Casemiro's foul on Akanji, which was clearly wrong, but it can give handball for an accidental brush off the fingers that nobody even saw as handball? A complete farce.