Onana enjoyed his best performance in a Red Devils jersey so far during Tuesday's 1-0 Champions League win over Copenhagen, where Harry Maguire's header got Erik ten Hag's side off the mark in Group A.
However, Man United were forced to endure a heart-in-mouth moment in stoppage time, as Scott McTominay's high boot in the penalty area gifted the Danish champions the chance to draw level from 12 yards.
Jordan Larsson stepped up to the mark, but Onana dived to his left and stuck out a strong hand to turn the striker's effort behind, and he was immediately mobbed by his teammates before the final whistle sounded.
Following the Cameroonian's stoppage-time heroics, Garnacho posted a photo on X - formerly known as Twitter - of Red Devils players swarming Onana, accompanied by two gorilla emojis.
Garnacho's post - which has since been deleted - sparked huge controversy, and the FA are now understood to be investigating the use of the emojis and have asked for the South American's observations.
Garnacho could potentially be handed a fine or suspension if his post was deemed to have caused offence, as was the case when Bernardo Silva compared former Manchester City teammate Benjamin Mendy to a character on a brand of chocolates in 2019.
Both Silva and Mendy affirmed that the post was a joke between two friends - which the FA accepted - but the former was still fined £50,000 and banned for one match due to the content being "insulting by reference to race, colour and ethnic origin", which brought the game into "disrepute".
Following the furore surrounding Garnacho's use of gorilla emojis, Onana took to Instagram to play down the incident and affirm that the winger should not face any consequences.
"People cannot choose what I should be offended by. I know exactly what [Garnacho] meant: power and strength. This matter should go no further," the goalkeeper wrote.
Prior to his man-of-the-match performance against Copenhagen, Onana faced scathing criticism for a spate of error-strewn performances since joining the Red Devils from Inter Milan, keeping just three clean sheets in his first 12 games for the club.
While Ten Hag confessed after the game that the Cameroonian shot-stopper has sometimes flattered to deceive this season, he could not fault Onana's mental fortitude, telling the press: "He showed personality.
"He knows the levels and what his skills are and he hadn't matched his skills and could do better but it was a very good performance on Saturday and tonight. He is a very good penalty saver.
"There is a very good spirit in the dressing room, they are together, they fight together, they celebrate together and if we have setbacks they support each other. I think that spirit is always needed to be a successful team. The win was justified but it was a narrow escape."
Man United have risen to third in Group A behind Bayern Munich and Galatasaray thanks to Tuesday's success, and the Red Devils are now gearing up for Sunday's Manchester derby against Manchester City at Old Trafford.