The 19-year-old Garnacho included the emojis in a post featuring a photo of himself and Onana celebrating the Cameroonian goalkeeper's remarkable save of an injury-time penalty against FC Copenhagen on Tuesday.
Garnacho later deleted the post, but English media reported that the Football Association (FA) had been made aware of the post and contacted the Argentinian.
"People cannot choose what I should be offended by. I know exactly what Garnacho meant: power & strength. This matter should go no further," Onana wrote on the social media platform X, formerly known as Twitter.
Garnacho has made no comment about his deleted post or about being contacted by the FA.
United won the Champions League Group A match at home against the Danish champions 1-0, thanks to a header from Harry Maguire. However, Man U could have dropped two points if Onana hadn't saved Jordan Larsson's penalty several minutes into stoppage time.
