Jose Mourinho revealed Antonio Valencia was sent off in the friendly win against Real Salt Lake after he refused a request from the referee to substitute the Manchester United full-back.
Romelu Lukaku scored his first United goal as substitute Valencia saw red in Monday's 2-1 pre-season victory at Rio Tinto Stadium.
Leading 2-1 at half-time, Valencia was introduced but he lasted only 15 minutes after a late tackle from behind on RSL's Sebastian Saucedo, who was involved in a heavy challenge on Juan Mata only moments earlier.
Referee Allen Chapman initially awarded a free-kick but the MLS official then suddenly brandished a red card after speaking with Mourinho on the sidelines.
"I think the delay was because the referee asked me to change the player and I didn't," Mourinho told reporters.
"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.
"Mata has a big ankle but I hope nothing important. A few days, a week maybe."
It was a memorable outing for Lukaku, who opened his account for United after debuting in the 5-2 rout of LA Galaxy last week.
After cancelling out Luis Silva's opener, Henrikh Mkhitaryan then teed up former Everton striker Lukaku for the game-winning goal six minutes before the break.
"I think it's just nice for him, it's not important for me," Mourinho said. "Before he scored the goal I told him 'I love everything you do on the pitch - don't be worried about scoring or not scoring'.
"He gets behind people, works and presses well. I like everything he does. It's easy to feel it because he was my player four years ago his evolution has been great. He's now a top striker. The goal is just a detail, not important."
