United led 2-0 after 11 minutes of Sunday’s quarter-final but Chelsea hit back to level the score in the second half and the hosts were left hanging on in the dying stages.
Chelsea could have snatched a winning goal in the last minute of normal time, when Juan Mata cushioned a high ball and stepped inside Jonny Evans before blasting a shot at goal.
However, De Gea intervened, spreading himself to divert the ball wide of his right-hand post and thereby keeping United in a competition they last won in 2004.
“It was a very good save,” said Ferguson.
“I saw the replay and it was terrific at a very important time, because there was no doubt we would be out if they had scored at that time."
United had appeared to be cruising into the semi-finals after early goals from Javier Hernandez and the recalled Wayne Rooney put them in control on a wintry afternoon at Old Trafford.
Their poise disappeared in the second half though, as goals from Eden Hazard and Ramires brought Chelsea back into the tie, and Ferguson said Tuesday's sapping Champions League loss to Real Madrid was to blame.
“I can’t be critical of the side really, as the running they had to do on Tuesday against a top side in Real Madrid had a bearing on this result, there is no doubt about that,” he told in-house television channel MUTV.
second-half comeback on the night was vindication for interim manager Rafael Benitez, who has endured a torrid time at the club since replacing the sacked Roberto Di Matteo in November.
Birthday special: 14 Bollywood actors who made it big on TV
Photos: Vaani Kapoor, Poonam Pandey, Sana Khan at fashion event
Photos: Rishi Kapoor, wife Neetu, daughter Riddhima at book launch
Photos: Sunny Leone, Shraddha Kapoor, Shriya Saran at Mumbai airport
How Angelic! 10 stunning pics of birthday girl Angelique Kerber