The winger posted two gorilla emojis alongside a picture of Onana to celebrate the keeper's Champions League heroics against Copenhagen on Tuesday night. He later deleted the post.
The FA have contacted the youngster and he player could face sanctions if the response is not satisfactory.
Amad showed support for Garnacho on his Instagram profile by uploading the forward's photo along with flexed bicep and love emojis.
@amaddiallo19 InstagramNext Match
Onana has also defended the youngster, clarifying Garnacho's intentions and insisting that the "matter should go no further." The goalkeeper's message, as well as Amad's support, may prevent Garnacho being hit with an FA ban.
United have not imposed any internal sanctions on the player and he should be available for selection when the Red Devils host Manchester City on Sunday.