There was a lengthy delay between Valencia committing the challenge and the American referee dismissing the United right-back, as Mourinho was spoken to.
Once the discussion was finished, the official walked over to Valencia and expelled him.
"I think the delay was because the referee asked me to change the player and I didn't," Mourinho explained. "Because I don't agree with the card. Because Saucedo, I think is the name of the boy, he was really aggressive during all of the second-half, with what happened to Mata and some other actions where it was a bit dangerous, which I understand.
"Young people, enthusiasm playing against Manchester United, but you come with the mentality of a friendly match and in the end you find yourself in a different mentality.
"But it's pre-season, no problem and also a good experience for us to play with one player less."
Valencia will not serve a suspension for his red card.