Grimsby Town, a team from the lower division, won the match in a penalty shootout, despite fielding an ineligible player.
The ineligible player, Clark Oduor, was a 26-year-old midfielder loaned from Bradford City.
Oduor came on as a substitute and even took a penalty kick during the game, which he missed.
The English Football League (EFL) investigated the matter and imposed a £20,000 fine on Grimsby Town, with half of the fine suspended until the end of the season.
Manchester United had the option to appeal the result, but they have chosen not to do so.
