Cech made one memorable save in particular, clawing away a point-blank header from Javier Hernandez in the second half to keep Chelsea in front.
And Ferguson believes the Czech Republic stopper was the difference between the sides as Chelsea progressed to the semi-finals.
"Cech's save from Chicharito was unbelievable," Ferguson said in the Metro. "He made a similar save in the first game from one of his own defenders. Had we got that goal then I think we would have been okay.
"It may have gone to extra time but we wouldn't have needed to go chasing it."
Chelsea squeaked through with a 1-0 victory courtesy of a stunning second-half strike from Demba Ba. The Senegalese forward hooked home a perfect Juan Mata pass to put the Blues ahead, and they held on to book a semi-final date with Manchester City.
United will now turn their attentions to the Premier League, and they've got their own Manchester City fixture next week, when they take on the defending league champions at Old Trafford on Monday.
But they had opportunities against Chelsea to keep hopes of a double alive, with Robin Van Persie in particular spurning good chances.
"One he really should have scored but he headed over," said Ferguson. "It was disappointing because we are better than that. Over the 90 minutes, only Antonio Valencia reached the standard we expect."