Rashford reacted angrily to a challenge from Phil Bardsley, and thrust his forehead into the Burnley player's face.
The situation escalated, and referee Jon Moss booked Bardsley before showing Rashford red, just 10 minutes after he came on as a substitute.
September 2, 2018
The 20-year-old now faces a three-match ban for violent conduct.
Mourinho claimed that there was some clever handling of the situation from former-United player Bardsley:
"I would say it was naive," Mourinho said.
"It was a kid and a very experienced man. Bardsley's been in the game for 20 years and Marcus is a naive boy."
"Moss had a good game, a good performance, really. I think the same as the two assistants."
"In that action, the only thing I can say is that probably the experience of Bardsley against the impulse and naivety of a guy that is in top football for about three years but is still a young kid."
"Harsh? I don't know. It could be a yellow card for both and keep going."