United's Ashley Young posted a tweet after the tie with him holding the coin towards Gunners fans and the message: "Heads we win, tails you lose."

The defender picked up the object following a melee involving opposing players towards the end of the game.

United won the fourth-round match 3-1 at the Emirates.

The spat started off with Gunners left-back Sead Kolasinac and United striker Marcus Rashford squaring up on the touchline. Jesse Lingard and team-mate Young also got involved along with several Arsenal players before Kolasinac and Rashford were booked.

FA investigations into incidents such as coin-throwing involve seeking observations from the clubs and, if needed, the police.

The governing body is expected to work with Arsenal to identify the culprit and make sure they are dealt with by the club. Disciplinary charges would only follow in serious cases if the FA determined that the club concerned did not do enough to prevent or deal with the misconduct.