Ferguson signed De Gea in a £17m deal from Atlético Madrid in 2011, and the Red Devils manager believes the young goalkeeper is steadily improving despite recent setbacks.
“David has shown his character because he has taken a lot of criticism, sometimes unfair criticism,” said Ferguson.
“But what we at United are good at is developing people, bit by bit. It’s like a young child learning to walk. He moves forward, he wobbles. He gets up, wobbles, gets up again and now he’s walking.”
De Gea, 22, believes his Spain captain Iker Casillas will be sorely missed by the home side after the Madrid goalkeeper suffered a fracture in his left hand last month.
“Every time Iker Casillas is not there, he is a great loss,” said De Gea.
“It is a great loss because he is the captain of Real Madrid and Spain. But Real have two other keepers who can play as well as Casillas does.”
• Cristiano Ronaldo spent six seasons with United between 2003 and 2009. He scored 118 goals in 292 games, winning one Champions League and three Premier League titles, as well as one FA Cup and two League Cups.
• The sight of Ronaldo facing United for the first time since his departure adds another layer of intrigue to a contest pairing clubs who have met in four previous European Cup ties – with the winners going on to lift the trophy on three occasions.
• Madrid came out on top in three of the clubs’ four previous encounters – in the semi-finals in 1956-57 and quarter-finals in 1999-2000 and 2002-03 – with United prevailing in the 1967/-8 semi-finals.