Garnacho was the unlucky individual who got substituted for tactical reasons after defender Patrick Dorgu was sent off in the first half of United's 3-2 win over Ipswich Town on Wednesday.
But instead of sitting on the bench and watching the remainder of the half, Garnacho headed straight down the touchline and into the tunnel at Old Trafford.
The young Argentine also posted a picture of himself looking disappointed on the pitch on social media after the game, apparently to voice his displeasure at the decision... despite United actually winning the match.
In his post-match interviews Red Devils boss Ruben Amorim confirmed he would speak to the forward about the incident.
Former Wales international and talkSPORT pundit Dean Saunders took a stronger stance on the issue and warned Garnacho his attitude was 'going to ruin his career' if he kept it up.
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But speaking at his press conference on Friday, Amorim confirmed the reason behind Garnacho's mid-match dash was nothing sinister.
However, the 20-year-old would still be punished given the negative perception it gave off as Amorim looks to set high standards - by buying the entire team dinner.
"He came to me," Amorim said.
"Was funny because the next day he came to me to my office. I did some investigation and he went to the dressing room, changed his clothes because he was wet, he watched the game not on the bench and at the end of the game he was there and went home. So there is not an issue there.
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"He's a young kid that will learn. The important thing was the next morning he was speaking with me. I think we need to see that small things like walking out onto the pitch - because (Joshua) Zirkzee did the same thing and the context was different - but it was the same thing.
"We have to understand the situation, the context, the age of the players, of course they have to be better.
"They have to understand perception here is really important. But you can deal with that the next day and move on so let's just move on.
"But I told him at Manchester United, everything is important and the perception in a big club is really important. So he is going to pay a dinner to all the team and that's it.
"I think he (Garnacho) understands. But, like I said, [the players] read everything, all the small things. And I know the players quite well, I played for quite a while, I understand they are frustrated. He was subbed, he was playing well and has to go off in that moment of the game.
"It's hard to deal with everything but they have to deal with everything. I just try to help them be better footballers."
Despite Garnacho and Amorim clearing the air regarding what happened against Ipswich, the Argentinian is no guarantee to feature for United's FA Cup fifth round tie against Fulham on Sunday.
"He has a knock," Amorim said.
"When he went to shoot, was blocked almost inside the box. He has a knock, I expect for him to be ready but we will see.
"I have to wait until tomorrow."
A win over the Cottagers on Sunday would be United's third in their last five across all competitions.
The Red Devils have already beaten Fulham twice this term, with both league fixtures ending 1-0.
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Victory for United on Sunday would also be the second time in the last three campaigns the English giants have beaten Fulham three times in a single season.
The two teams met in the quarter-finals of the FA Cup during the 2022/23 season at Old Trafford, as United ran out 3-1 winners over a nine-man Fulham.
Going to ruin his career! Alejandro Garnacho slammed for storming off down tunnel after being hauled off by Ruben Amorim