David de Gea's introduction to English football has been challenging for everyone concerned and, in the worst moments, fairly harrowing for a young man in a new country, stepping into an environment that is notoriously knee-jerk and unforgiving when a new player with a large price tag has any form of teething problem.

De Gea's sympathisers have argued long and hard that it was perfectly normal that a goalkeeper of 20 years of age, playing in a foreign city, with little grasp of the language, would inevitably be vulnerable to the mistakes that briefly threatened to engulf him a couple of months ago. It is a reasonable line of defence and it is probably only fair to note that many of those praising De Gea for his recent performances were the ones in the stampede to judge him in those difficult moments when he was costing his team potentially crucial goals.

Yet Sir Alex Ferguson has admitted himself he did not expect De Gea to be quite so vulnerable. Nobody sensible has ever said De Gea would never make it and back in August, when he was dangerously close to being a liability, a clear and legitimate debate did exist about whether Ferguson could continue to play someone who looked so raw and nervous when United had assignments against their five main rivals in the first nine games. As it has turned out, Ferguson reiterated his trust in his new signing and has been rewarded in full as the concerns surrounding De Gea's form and state of mind have made way for something considerably more in his favour.

De Gea has got it out of his system and turned down the volume on the gloating cries of "dodgy keeper" that accompanied him throughout his first excursions in United colours. Sometimes in football there is no substitute for guts and De Gea was United's outstanding player at Liverpool last weekend, as if immune to the pressures of wearing that little red devil and his trident in front of the Kop. He has, in short, emerged with distinction and deserves a high form of praise for his resolve and mental fortitude, because without those qualities he could easily have suffered a serious loss of nerve.

The Spaniard is still unusually slight for a goalkeeper and the truth is he does not always exude the sense he is in total control of his penalty area. But he has other attributes and there have been enough illuminating moments to soothe the memories of those errors in the Community Shield and the opening weekend of the season at West Bromwich Albion.

De Gea looks more confident and, in turn, his team-mates seem more confident in him. De Gea has shown he has the agility of a gymnast, but a top goalkeeper also needs resilience and unlimited courage. Above all, he must have confidence because, without it, he is finished. De Gea is starting to show he has all of the above. "He knows that to play in this shirt, first of all you need to enjoy it," Javier Hernández says. "But also he is aware that it is a big responsibility. He is really handling it well."

Joe Hart is four years older, firmly established as England's first choice, and plays with the calm and assurance that comes from being old ahead of his years. He is the taller man, at 6ft 4in, whereas De Gea's official height, 6ft 3in, can look a misprint sometimes. Hart has a penalty-box presence his rival sometimes lacks and is now so consistently reliable, particularly when it comes to high balls, crosses and claiming his territory, he can be taken for granted sometimes and probably does not get the praise he deserves.

De Gea is learning fast and can develop that part of his game – "He will tell you himself he has loads of things to learn," Rio Ferdinand says – but it would be unrealistic to think the mistakes are over because they are probably not. Hart was the same at 20, showing rich promise but very much a work in progress, and it is only the last couple of seasons, under Roberto Mancini, that he has truly made the position his own.

Mark Hughes, Mancini's predecessor, signed Shay Given on the basis that he thought Hart made good saves but not outstanding saves. Hart was moved to Birmingham City, where he quickly set about showing that was a fallacy, winning the club's player of the year award and earning a place in the PFA team.

At 24, he is now a mandatory first-team pick for England while young enough to be regarded as someone who can still improve. Mancini, for example, has stopped more than one training session to address Hart about the importance of quick distribution, turning defence into attack, and understanding that when he gets the ball in a congested penalty area it is a time when the opposition can be vulnerable.

De Gea already has that in his artillery and is unusually precise with his kicking and accomplished enough with his feet to challenge Ruud Gullit's theory that "a goalkeeper is a goalkeeper because he can't play football".

Hart, though, now ranks as "one of the top five goalkeepers in Europe" according to Mancini, and even the slightest lapse, such as Villarreal's goal at the Etihad Stadium on Tuesday, is rare. Costel Pantilimon, the Romania international, was signed on loan from Timisoara in August, but as backup rather than serious competition. Stuart Taylor, now third in line, has one of football's more unorthodox roles, particularly as City do not operate a reserve team. Taylor's last game was 21 months ago.

Ferguson's policy has been slightly different, with De Gea occasionally making way for Anders Lindegaard or Ben Amos. As always with United, the boundaries are blurred when it comes to whether this is a case of being rested or dropped. Ferguson, however, would almost certainly say the former. De Gea has made more saves than any other Premier League goalkeeper so far and has the best save-percentage (85.7%), ahead of Hart (79.3%).

Hart remains the safer pair of hands but United are no longer yearning for Edwin van der Sar's calm and that in itself is a triumph for the young Madrileño, three weeks shy of his 21st birthday.