Referee Roberto Rosetti sent off Fletcher after a challenge on Cesc Fabregas, but replays show that there is no doubt that the Scottish midfielder got the ball.
Rosetti was derided by fans of both sides after the 1-3 win to Manchester United at the Emirates Stadium today.
United boss Sir Alex Ferguson said: “You can see the ball has moved in a different direction. It’s terribly unlucky and we can’t appeal it.
“The referee is one of the best in Europe, he may look at it and have a view on it but there’s nothing you can do about it.
“In all the big games, Fletcher has always been a key player for us. I couldn’t tell you what our team in the final would be – but you’d think Darren would have featured in it.”