The 20-year-old took part in United's 2-1 victory over Champions League winners Barcelona in the early hours of Sunday morning as part of their pre-season tour of the USA
"Replacing Edwin van der Sar is very difficult; he was a great goalkeeper," said Ferguson. "But the benefit we have with De Gea is that he is only 20.
"The qualities that stood out for us are his great composure and use of the ball. He is quick and agile, which is something we banked on.
"When we assessed him I thought he was the right goalkeeper to take," he added.
The Premier League champions begin the defence of their title on Sunday, August 13 away at West Bromwich Albion.