The forward was dismissed following a VAR review at Parken Stadium, with the visitors leading 2-0 at the time.
Erik ten Hag’s side would go on to lose 4-3 and now sit bottom of Group A on three points, one behind Galatasaray and Copenhagen, with their place in the last 16 in jeopardy with two matches remaining.
After two early Rasmus Hojlund goals, United found themselves down to 10 men when Rashford challenged Elias Jelert in the 42nd minute.
Rashford positioned his left boot across his opponent in a bid to shield the ball, but the Danish player was too quick and placed his left leg ahead of his opponent, leaving the United star’s studs trapped on his shin.
Despite an apparent lack of intent from Rashford, referee Donatas Rumsas then sent the player off with a straight red card for serious foul play after a lengthy VAR delay to review the challenge.
Uefa stated on their website: “the forward is dismissed after catching Jelert.”
Rashford is the second United player to be dismissed in the Champions League this season after Casemiro was sent off for two bookable offences against Galatasaray at Old Trafford last month.
Ten Hag was furious with three key decisions and labeled the decision "harsh” with Rashford now set to serve a one-match ban, although Uefa can elevate this to two matches: “In case of serious offences, the Uefa Control, Ethics and Disciplinary Body is entitled to augment this punishment, including by extending it to other competitions.”
Aaron Wan-Bissaka saw his one-match ban for a straight red against Young Boys in 2021 upgraded to a two-match ban by Uefa for “rough play”.
"The red card changed everything," Ten Hag said. "We then conceded two goals before half-time that should never have counted -- the first was offside, their player was stood in front of André Onana, and the penalty [for the second] was very debatable.
"The red card was harsh. [Rashford] went for the ball, but the referee had such a long time to make it a red card. When you freeze it, it always looks worse. They took so long to make a red card of it.
"I'm very disappointed with the decisions, the game was never meant to be like this. Decisions have to be made, wrong decisions can be made, but three such tough decisions [went against us]."
While former Man Utd player Owen Hargreaves insisted Rashford deserved to stay on the pitch: “No, not in a million years. Marcus is just trying to put his leg in front of the ball. He’s not even looking at the ball. When you see a still as a referee, it looks horrendous. It’s not malicious, it’s not serious foul play.”
After United travel to Galatasaray in Istanbul later this month, they then host Bayern Munich at Old Trafford in their final game and likely require four points from the final two games to qualify ahead of their Turkish rivals and Copenhagen.