De Gea, 20, completed a big-money move from Atletico Madrid this summer following the retirement of Edwin van der Sar, and it was expected that he would instantly become the club's first-choice goalkeeper.
However, with 27-year-old Denmark international Anders Lindegaard having joined the club in January, De Gea faces a battle for his place.
"The only area where we may have to be patient is in the goalkeeping position," Ferguson said. "De Gea is only 20, he's young. He's played in the Spanish league for a couple of years, but it's not the English league.
"I think he'll need time to settle in, but the potential is enormous. It's absolutely unbelievable, the potential the boy has got. We've also got Anders Lindegaard, so we may get a challenge for them both to accept, but in time the boy De Gea will make it his position, there's no doubt about that."