It was after the match against Southampton at the end of November that Cavani thanked a supporter with a comment that was perceived as racist.
"Edinson and the club were clear that there was absolutely no malicious intent behind the message, which he deleted and apologized for as soon as he was informed that it could be misinterpreted," United said in a statement.
By January 4, Cavani must submit a response to the accusations from the FA.