The Reds opened the scoring at Anfield through Daniel Agger, although Park Ji-Sung ensured the teams would head into the break level pegging.

However, with just two minutes remaining of normal time Dirk Kuyt fired home the decisive goal to send the Merseysiders through to the fifth round at the expense of the Red Devils.

"I don't know how we lost it," Ferguson told the club's official website. "It was a really good performance from us.

"That can happen in the FA Cup though - you can have one bad break against you and you lose the game.

"It's a sudden death situation which we've experienced many times over the years and anything can happen. It's a bad blow though because we didn't deserve that.

"I'm pleased with my players. They performed very well, dominated the game and played good football.

"They didn't deserve to be beaten at all. We maybe should have won the game - we were the better team.

"But that's FA Cup football I'm afraid and it's not the first time it's happened. To lose it is a devastating blow."