The Argentina international added two gorilla emojis to a social media post with a picture of Onana and his team-mates celebrating the Cameroonian's last-gasp save from Jordan Larsson's penalty, which secured United's vital 1-0 win over Copenhagen in the Champions League on Tuesday.
Garnacho was advised to edit the post on X - formerly known as Twitter - as it could have been interpreted as having racial connotations. He removed the emojis and then deleted the post altogether although it had been captured by social media users and posted on Reddit.
The post could lead to Garnacho being hit with a suspension based on the FA's previous actions on racially sensitive posts.
Twitter.com/agarnacho7In 2020 United's then-striker Edinson Cavani was suspended for three matches for a writing 'Gracias Negrito' to a friend on Instagram who had congratulated him for scoring. The term 'Negrito' is sometimes used as an affectionate greeting in Uruguay but the post fell foul of the FA's guidance on racially sensitive social media posts.
In 2019, Manchester City midfielder Bernardo Silva was banned for one match and fined £50,000 by the FA for a racially sensitive post about his then team-mate Benjamin Mendy. Silva had tweeted a photo of Mendy as a child alongside an old image from Spanish company 'Conguitos' that has been condemned for racist undertones.
Similarly, Napoli ran into trouble with star striker Victor Osimhen last month after a Tiktok video posted by the club's social media team appeared to mock the Nigerian.
Garnacho is not understood to be in any immediate trouble with United over the post as he was not aware of the connotations and deleted it.
United declined to comment on the post.