It was the height of the foul but he was clearly slipping as he reached. It’s a yellow all day long, but a really harsh straight red. To be fair United deserved to be a man down for coming out that flat at home after a poor mid week showing.
Despite what the color analyst says in the below video, the height wasn't much of a factor in the call, IMHO. Fernandes would have been shown a card if he had hit Maddison in the ankle area. For several years, FIFA, US Soccer, etc., in their ongoing training of referees, ongoing videos sent out to refs, has really stressed that cleats to the body on a challenge should almost always result in a red card. About the only exception is a challenge where one or both of the players ends up rolling, and in contact after the initial challenge, there is an inadvertent cleats to the body as the player(s) are rolling around.
The red card may seem harsh, in that Fernandes didn't hit Maddison all that hard with his cleat. But that's not really something the ref is supposed to factor into the evaluation.