De Gea lost his place to Anders Lindegaard after spilling a cross in the 3-2 win against Fulham last weekend.
Ferguson confirmed that the mistake forced him into making a change for the trip to St Mary's.
The United boss told Sky Sports News following the narrow victory against Saints: "I think young David made a mistake last week and he knows it.
"With the form he's been in, he's been making some fantastic saves, but one error like that could have cost us the game.
"It's just a learning process for him and he'll be back in a couple of weeks' time after the international break."
Lindegaard also conceded two goals in his first Premier League appearance of the season as United came from behind to snatch a 3-2 win against Saints thanks to a Robin van Persie treble.