Only a crucial late save from De Gea kept United in the competition.
"We could have been four up in the first 25 minutes," said Ferguson.
"Wayne and Javier's goalscoring has been great this season.
"We needed that because by the end of the first half I detected our legs were starting to go.
"It was a very good save by David de Gea near the end. If they had scored then we were out there would have been no doubt about that."