The Spaniard joined United at the start of last season and received criticism for his early performances, before being rotated with Anders Lindegaard.
Ferguson insisted that the 22-year-old is the definite number one now and hailed his progress.
"David is number one now, definitely," Ferguson said. "He has really improved this season, especially with the physical part of his game.
"If you remember his first game at West Bromwich Albion last season, he got absolutely battered. But you can see the maturity in him now. The physical stuff doesn't bother him. He can handle it.
"We have to thank our goalkeeping coach Eric Steele for his progress this season. Steely kept reminded David that he is a top player and he has responded to the physical part by putting on almost a stone in pure muscle."
De Gea and United face Aston Villa on Monday evening and could secure the Premier League title if Manchester City lose to Tottenham Hotspur tomorrow.