De Gea produced a wonderful performance to earn United a goalless draw at Sevilla in the opening leg of their Champions League last-16 tie, pawing away Luis Muriel's point-blank header in a moment of brilliance on Wednesday.
It further strengthened De Gea's case as the best goalkeeper in the world amid constant speculation of a possible move to Real Madrid, however, it has not been all smooth sailing for the Spain international at Old Trafford.
De Gea initially struggled with the physicality of the Premier League after being lured from boyhood club Atletico Madrid as a baby-faced Spaniard seven years ago.
"We were talking before, when he first came, even as players we were sitting there with doubts really," Ferdinand told BT Sport .
"A young kid just coming to Manchester United, and we needed to win every year.
"How were we going to do that with a young kid, who was inexperienced and looked like he was lacking confidence?
"But you look where he is today and that's a sign of someone who has worked hard to get where he needs to get."
8 - David de Gea has made eight saves against Sevilla, the most for a Manchester United goalkeeper in a Champions League game since Edwin van der Sar vs Barcelona in May 2011 (8). Hero. pic.twitter.com/OpxZQSzK9p — OptaJose (@OptaJose) February 21, 2018
De Gea and United are back in action on Sunday, when they welcome Antonio Conte's Chelsea to the Theatre of Dreams.
Jose Mourinho's side will welcome Sevilla to Old Trafford on March 13 for the second leg of their Champions League tie.