The shot-stopper joined the Red Devils from Atletico Madrid for £18 million following the European Under-21 Championships, which De Gea's Spain won, with the 20-year-old making his debut in United's pre-season friendly against Chicago Fire.
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De Gea starred in the Red Devils' 3-1 victory, making a string of saves with United 1-0 down against the MLS outfit.
But Ferguson admitted that, while he has no issues about the goalkeeper's age, the language barrier could yet pose a problem.
"The only problem with De Gea at the moment is that he doesn't speak any English," he said in the Sunday Mirror. "We have got him working on certain calls and shouts with the defenders.
"I'm not worried about his age. I looked at Petr Cech when he was a teenager at Rennes and when we dismissed him because of his young age, he then went to Chelsea and it was clear we made a mistake.
"When we were scouting De Gea, we dismissed his age and focused on what qualities he could bring and he has a lot going for him.
"At the moment, he hasn't got a great physique, he's tall and wiry but we will work on that."
And the man tasked with improving De Gea's game with the Premier League champions, goalkeeping coach Eric Steele, is confident that the Spaniard will prove to be a worthy successor to Edwin van der Sar - but not without a lot of hard work.
"Did you see Edwin when he was 20? There is work to be done, but no player is completely finished at 20. Edwin didn't come to England until he was 34," he said.
"David is still getting to know players and the language, but he can fit into the team because he's got this calmness.
"He's very calm and mature on the ball for his age, which we've been used to with Edwin, but if we're going to play the Man United way, we need a keeper who can do that.
"David has a tough act to follow, but we always knew that and that's why we took our time."
