I haven't seen this particular incident but I do have some sympathy where the contact with ball or the ground causes the foot to rise and catch the opponent higher up. The question for VAR though is "was it a clear and obvious error" not necessarily "would I have given a RC"? That said, Marriner, who on a previous occasion has stuck with his original (non-penalty) decision was obviously persuaded in this case.
Having watched the 2nd half and seen the incident only once, is it generally agreed that Fernandinho's what we might politely called a challenge only warranted a YC. I'm tempted to use Hendo's conjecture used in his many swipes at the alleged controversial decisions that United get, "I wonder what the decision would be had that foul been by a Leeds player". But as I say, maybe the YC was perfectly correct.
Definite red for Cooper, definite yellow for Fernandinho. Unbiased view. 😛