Manager Ferguson has backed the summer signing from Atletico Madrid to be a success in the Premier League but believes that he has a lot to learn.
"David should have done better there. I think he's lost a bit of concentration there, but welcome to English football because he took a battering in the second half," Ferguson told Sky Sports News.
"My recollection of goalkeepers coming from Europe is there's a learning process, it's a different game.
"Peter Schmeichel was the same when he came. [It was] Wimbledon in his first away game on a Wednesday night and they gave him a torrid time.
"But Peter came on to become the greatest goalkeeper of all time, possibly. The boy is 20 years of age, you suspect a learning process to be in operation at the moment and he'll see it through."
United reportedly paid Atletico £18m for the Spanish under 21 international.