Many judges believe it was Fletcher's finest in an United shirt as Ferguson chose the Scot ahead of Paul Scholes for the big match.
"It was a big decision because Scholes' performance against Fulham was vintage," Ferguson told manutd.com. "If the game had been on Wednesday I would have played him. But I had to ask the question with it being three days after the Fulham, especially given the time he has been out injured and his age.
"Darren came in to do a job, which he did very effectively. He's not easy to play against. I wouldn't like to play against him. He has such a long stretch, which means he can close players down quickly and he wins possession without conceding many fouls. That was important to us, because Juninho's ability from set-pieces could have been a threat to us."
The 24-year-old really grasped his chance in the 4-0 FA Cup win over Arsenal. Before then he had made just six starts all season - in part due to injuries. Ever since, he has been outstanding.
"Darren emerged from the Arsenal game with great credit, and since then he's come into the frame," said the boss. "It's a great credit to the lad and the patience he's shown.
"He shows something we always admire at this club: true professionalism. That's why you give players good contracts. You hope they get the first part right and obey the laws of being a professional. He is a very good example of that."