Jermaine Beckford's 19th minute goal gave League One leaders Leeds, 43 places below English champions United, their first Old Trafford victory in 29 years during what Ferguson readily conceded was "just a bad" display by his side.
Related ArticlesMan Utd considers £600m bond issue to refinance debtsLeeds shock Old TraffordMan Utd-Leeds classicsLeeds United then and nowNewcastle confident of signing Jermaine BeckfordSport on television"The preparation for the game was very good, but I'm shocked by the performance," Ferguson said.
"We didn't start right and Leeds did start right. They got their goal and had something to hold onto," the 68-year-old Scot added.
"They fought like tigers, and we expect that with any team coming to Old Trafford for an FA Cup tie. It's a disappointment."
Reflecting on Leeds's decisive goal, Ferguson said: "We did speak about Beckford's pace up front.
"We were caught napping really, it was a bad goal for us to lose. The whole performance in the first-half was bad. We never got going and the quality of the passing - the whole performance was just bad.
"It's one of these things. We're only human and sometimes you have performances that surprise you. But we didn't expect that today."
Ferguson added: "I don't think any of the players can say they had a good day. Maybe only (Antonio) Valencia, when he came on, can say that. Even then it took us about ten minutes to get the ball to him."
The United boss rested several players ahead of next week's League Cup semi-final, first leg, tie against local rivals Manchester City and Ferguson said: "We have to get this result out of our system as quickly as possible.
"We have a semi-final on Wednesday and a lot of these players won't be playing. You have to view this performance in the right light.
"We'll make sure we're ready for Wednesday now. We had a team in mind, but there will maybe be a few changes for that."
Leeds manager Simon Grayson hailed his "outstanding" side after they produced one of the great FA Cup shocks.
"It was a good ball from Jonathan Howson and Jermaine got in there and nine times out of 10 he finishes them off and that's what he did," Grayson told ITV of Beckford's goal.
"He gave us an opportunity to go on and win the game from there and with a bit of luck that's what we did," the lifelong Leeds fan added.
"Full credit to the players who were outstanding today but it's what they've been like all season."
Grayson, recalling how Brian Flynn had been the last man to score the winner for Leeds at Old Trafford back in 1981, added: "I saw Flynny the other week and he reminded me he was the last player to score the winner at Old Trafford.
"I told the players before the game this was the opportunity to be a new hero. I'm sure we'll be celebrating in Leeds over the next few days and the players might have tomorrow (Monday) off then start ready for the Wycombe game at the weekend."