The Spaniard, who has made a number of goalkeeping errors since his arrival at Old Trafford in the summer, has faced competition from Danish keeper Anders Lindegaard for the starting spot all season.
However, the Scot insists that the former Atletico Madrid star is their man moving forward.
"Yes, I have a No.1 keeper," said Ferguson. "De Gea is the long-term prospect – there is no question of that. Anders has done very well but David's potential is obvious.
"I know who I am playing in goal on Sunday against City. I have been leaving David out of games quite regularly – there have been opportunities for Anders playing in Europe and some league games.
"But because we lost two bad goals against Blackburn doesn't mean all of a sudden we are neglecting that part of both of them playing games."
De Gea has made 14 appearances for Ferguson's side after arriving at the club for a fee of over £17m last summer.