The United manager saw a Jermaine Beckford goal knock his club out of the competition at the third-round stage for the first time since 1984 and give Leeds, the only giantkillers in the third round, their first win at Old Trafford since 1981.
Ferguson, who rounded on referee for playing only five minutes added time, criticised his failing stars after their defeat by League One opposition. He warned them all they face the axe ahead of Wednesday's Carling Cup semi-final first leg against Manchester City.
Ferguson said: 'I didn't expect that. I was shocked at the performance. We never got going and the quality of passing the whole performance was bad.
Anger: United boss Sir Alex Ferguson cannot bear to watch as his side waste a chance at Old Trafford
'I don't think any of them can say they had a good day. Only Antonio Valencia did OK when we got the ball to him, but it took us about 10 minutes to do that after he came on.
'We have a semi-final on Wednesday and a lot of these players today won't be playing.
'I had the team in mind but there may be one or two changes now.'
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Ferguson saw his team sunk by Beckford's 20th-minute goal and the result was the only one of the weekend in which a lower-placed team beat one of a higher standing.
United's exit leaves their record of 11 FA Cup wins vulnerable to being equalled by Arsenal, who have won 10.
Leeds' reward in last night's fourth-round draw was another trip to face Premier League opposition, Tottenham.
The United manager was critical of referee Foy over the five minutes' added time, saying: 'That's an insult to the game and to the players out there.'
These comments will interest the FA, given that Ferguson was given a four-match ban with two of them suspended for 18 months after criticising the fitness of referee Alan Wiley earlier in the season.
His assessment of Nemanja Vidic's reasons for limping out of the warm-up before the game also appear to hint at a problem in his relationship with the Serbian defender.
Asked why Vidic said he couldn't play, Ferguson said: 'I couldn't tell you what the situation is. I couldn't tell you at all.'
But this was a day to celebrate the achievements of Leeds United. Absent from the top flight for five-and-a-half seasons and brought to their knees by financial problems, Simon Grayson's team are top of League One and can now celebrate a famous victory.
Grayson said: 'We are immensely proud of our team and what they have done today.
Good job: Leeds boss Simon Grayson (left) celebrates with his backroom staff
'We told the players to do themselves justice and be proud of themselves. They certainly did that today. I told the players that if you could perform at Old Trafford somebody would be a hero.
'I saw Brian Flynn last week. He reminded me that he was the last player to score a winner at Old Trafford in 1981. Jermaine Beckford will get the credit now.'
Ferguson had previously never lost an FA Cup tie to a team from a lower league.
He added: 'Leeds had a far bigger appetite for the game. You need luck and they got it. But they deserved it as they played really well.'
After being drawn against Tottenham, Grayson said: 'It's a good draw again. We're up against a top Premier League team and a top manager and we'll approach that as we have done for the Manchester United game with the respect that they deserve.
'But we've got a few league games in between and that's our bread and butter.
'It's another fantastic draw and another game that could have quite easily been a Premier League fixture five or six years ago.' FA Cup fourth round draw: Leeds rewarded with trip to TottenhamSo who was up for the cup? Sportsmail takes a look at FA Cup third round attendancesJermaine Beckford's long road from west London to Leeds' FA Cup heroALL THE MANCHESTER UNITED FC