The Red Devils had to work hard to stay in the game against City, but took their chances late on.
United scored in the 88th and 90th minutes to get a 2-1 away win, which Rashford acknowledged despite not being in the squad.
The forward posted a photo that appeared to show him watching the game on television.
He added the message "Yesssssss! Love it lads," complete with heart emojis.
On dropping Rashford and Ajelandro Garnacho, manager Ruben Amorim told Sky Sports: “We try to evaluate everything training performance, game performances, engagement with teammates, push their teammates up. Everything is on the line when we analyse and try to choose the players. It is my selection. Simple.”
Amorim added: “I pay attention to everything, the way you eat, the way you put your clothes to go to a game. Everything. I make my evaluation and then I decide.
“There is a communication after the last training. They are alright. They had training this morning and I was there. They trained really well.”
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