Ferguson feels they now have one of the best young goalkeepers in the game.
"We worked hard to get the deal done because it wasn't easy," the manager told Inside United. "Atletico Madrid obviously regarded him very highly. But we managed to get it done and we're delighted he joined us.
"The past is the past and Edwin [van der Sar] is part of the past. He had an incredible career with us, he was an incredible goalkeeper. But, hopefully, the future belongs to David De Gea."