Demba Ba had put Chelsea into the lead before Cech pulled off a one-handed save to deny Javier Hernandez from scoring an equaliser, and Ferguson believes that the Czech Republic keeper was key to the Blues making the last four.
The Old Trafford boss told MUTV: "You have to say Cech's save from Chicharito was unbelievable.
"You need a bit of luck in the cup. He made a similar save in the first game from one of his own defenders.
"That was disappointing for us in terms of getting a little break. To be honest with you, I thought we were too rushed in that second half. We just lacked a bit of composure to create the proper openings."
Chelsea will now play Manchester City on April 14 for a place in the final.