Ferguson spent £18million prising De Gea away from Atletico Madrid last summer, only to see the Spain Under-21 star commit a series of gaffes that ended with him being dropped after a particularly sorry performance against Blackburn in December.
It is hard to believe De Gea is the same keeper now he has recovered his confidence after a series of outstanding displays, and Ferguson said: "I expected him to make a contribution the minute he arrived at the club. Since he was a young kid, our homework on the lad has always been really positive."
He added: "You can't deny the performance level and ability of someone who can get into the Atletico Madrid team at 18 and win the Europa League.
"That is why we went for him.
"Even Peter Schmeichel found it difficult coming into the English game, and he was 27.
"David made mistakes but we forgive that at 21."
De Gea will be in goal for Sunday's Barclays Premier League encounter with QPR, when victory would take United, for a couple of hours at least, eight points clear of Manchester City, who are due to kick off against Arsenal at the Emirates 30 minutes after the final whistle goes at Old Trafford.