The 71-year-old says the Real Madrid No.1 would not have pulled off the heroics Lopez did to ensure Los Blancos' passage to the Champions League quarterfinals
The Spain captain has spent much of 2013 sidelined with a hand injury sustained in Madrid's Copa del Rey clash with Valencia in January, prompting boss Jose Mourinho to buy Diego Lopez from Sevilla to deputize in his absence.
The 31-year-old has excelled since being granted a starting berth with the Liga giants and proved a major factor in Los Blancos' progression to the quarterfinals of the Champions League, and Ferguson says the outcome may have been different had Casillas been available for selection.
"I think Lopez saved them. I would have preferred Casillas," Ferguson told reporters.
"Iker wouldn't have saved the shots that the boy Lopez did. Going out to Danny Welbeck in the first half, going down at Robin van Persie's feet, coming out and whacking Nemanja Vidic in the head ... Casillas isn't that type of goalkeeper."
Casillas is scheduled to return from injury in the coming weeks but given the way Lopez has been performing in the skipper's absence, there is no guarantee that he would immediately return to the starting XI.
The draw for the Champions League quarterfinals will be made on March 15.