The Red Devils travelled to Chelsea on Easter Monday well clear at the top of the Premier League table and setting their sights on another league and cup double.
Those dreams were dashed at Stamford Bridge as a stunning volley from Demba Ba saw holders Chelsea book a semi-final date with Manchester City.
Ferguson admits his side had discussed the threat posed by Ba prior to the game and ended up paying the price for uncharacteristic dips in performance levels at both ends of the field.
"We were comfortable in the first half and I couldn't see them getting into the game or being any great threat to us. But the goal, as it always does, changed the game," he said.
"We spoke about Ba and the experience we had with him a couple of years ago at Newcastle when he scored exactly the same type of goal. We were caught napping, to be honest. It wasn't a good start for us right at the beginning of the second half.
"After that I think we lacked the composure to win the game and create proper openings. We had a lot of possession but didn't make the best use of it and they caused us quite a few problems on the counter-attack.
"They've got several players like Hazard, Oscar and Mata who sleep in little pockets and can cause you bother on the counter. Fortunately we didn't lose another goal, but I couldn't really see us scoring either.
"Robin van Persie had a fantastic chance and should have scored, but he put it over the bar. We had a couple of headers by the post and Cech made a fantastic save from Chicharito. But we just lacked composure when we had to chase the game and didn't play as well as we should have in the second half."
While frustrated to slip out of the cup, United can console themselves with the fact that they have surged 15 points clear of arch-rivals City in the race to be crowned champions of England.
"We must kick on," said Ferguson. "We're disappointed and the players are down, but we need to forget this and look forward to the game with City. A positive result for us next Monday will just about seal the title."