It is the look in his father's eye that Javier Hernández remembers most of all. "Something was different," he now recollects of that day early last year when they met outside the gates of the Estadio Jalisco, the home of Chivas de Guadalajara, and he found out life was never going to be quite the same again.

Talking about it now, Hernández still sounds almost disbelieving. First, his father, Javier Sr, told him he had some important news. Then he pressed a business card into his son's hand, and his words were simple and to the point: "There's someone who wants to speak to you."

Hernández was suspicious at first. "I had never heard of Jim Lawlor," he explains. "I didn't know who he was." The card told him it was Manchester United's head scout, but there is no trace of ego about the young Mexican, no sense that he always felt destined for a moment like this. "I didn't believe it. In Mexico, the agents put the badges of all the big clubs on their business cards. I was thinking: 'OK, another one of them, hey?' I turned to my father and said: 'Don't joke with me.' But that was when I saw him crying. That was when I knew. That was the moment I realised it was really true, that it really was Manchester United."

His smile, flashing those perfect rows of teeth, tells you everything you need to know about his feelings at that moment. Later, Hernández tells the story of being so disillusioned with football during one spell out of the team at Chivas he almost gave it up to return to a college education. He was "fed up", he says, and it barely seems credible he is talking about the same man whose eyes were sparkling as he sat inside United's training ground this week and talked of a year that has established him as an authentic superstar in the making.

What has happened in that time, Hernández admits, has surpassed all his expectations. In fact, they will all say the same at Old Trafford, from Sir Alex Ferguson down. "The idea was he would spend his first year getting to understand United, build up his body strength, get used to England," Ferguson said recently. Hernández agrees: "My first thought was to work very hard to play a little bit, maybe 10 minutes, in every game." Paul Scholes, though, saw something different. After Hernández scored in his first pre-season match, Scholes caught Ferguson's eye and uttered two words: 25 goals. "He's a good judge, Scholesy," Ferguson now says.

Hernández has 20, but he has managed that in only 26 starts (with a further 18 appearances). He has won a championship medal, with a Champions League version possibly next. He has relegated Dimitar Berbatov, the Premier League's joint top scorer, to the edges and he has done so with a vitality and brilliance that leaves many observers believing it should be mandatory he starts against Barcelona.

In Mexico, where what was supposed to be a year's supply of "Chicharito" shirts recently sold out in two months, that is certainly the expectation. "They are showing it live on television," Hernández says. "I read the newspapers back home and they are all supporting me and looking forward to the final. They have even changed the time of an international friendly so it doesn't clash." Mexico versus Ecuador was originally due to kick off in Seattle at 9pm British time. It has now been put back two hours – purely because of the Hernández factor.

He smiles shyly about that decision and, reflecting on his impact in Manchester, his modesty can be seen again, making certain to reflect the support network at Old Trafford. "When I say I'm at the best club in the world, I mean it. It's unbelievable the support you get here, the care they take to make sure the players are comfortable and happy. It's made me feel like I've been playing here for two or three years."

Others at the club would say the credit belongs to him. It helps, for starters, that he speaks perfect English and has a personality that has made him a popular member of the dressing room. But the coaches also speak of a man who wants to improve himself, an avid learner, someone who routinely arrives for training before the other players to fit in an extra half an hour in the gym.

"In the first training session I looked at myself and realised I needed more strength. I am not the tallest or the biggest, so I knew I needed to improve on that. That first training session, it was a bit of a shock, to be honest. Then, my first game, against Chelsea at Wembley [the Community Shield], I noticed the physical side even more."

Hernández, Ferguson has noted, will often stay back after training to work alone, and it is that kind of enthusiasm and determination that can make it so difficult to imagine how someone so obviously in love with his profession could once have contemplated giving up the game.

He tells the story with a slight trace of guilt. He was out of favour, he says, with the Chivas manager, Efraín Flores, and had gone two years without scoring a goal. "I wasn't getting the minutes I wanted, the coach wasn't playing me – I don't know why – and I was frustrated. My confidence started to drop. It reached the point where I was no longer enjoying football. I went to my family and asked whether I should carry on."

The message back was firm and to the point. "They told me I had devoted a lot of time to trying to make my dream come true and not to give it up. They said to keep fighting, keep focused and the most important thing was to keep enjoying the game because people all over the world want to be football players."

Would he have quit otherwise? "Probably, yes." It was this period of his life, he adds, that prompted him to go down on his knees before every match and pray, to "say thank you [to God] for helping me through it".

United started watching him in October 2009, though it was not until the following April they concluded a £6.5m deal shrouded in an extraordinary level of secrecy. "We knew for two months but my father and I were told we couldn't tell the family, our friends or anyone else," Hernández recalls. "It was hard. We are a big family, we are all very close, and we always want to talk about what is going on with each other. But we kept to their wishes. We told nobody."

United were desperate to stop news getting out to minimise the risk of being gazumped. In the end, the deal was concluded in Manchester with Hernández's mother, Sopapilla, and his sister, Ana, also let in on the secret – but no one else. "The rest of us were told they were going to Atlanta," the player's maternal grandfather, Tomás Balcázar, the first of three generations of the family to have played for Mexico in World Cups, recalled recently. In fact, Hernández and his father were watching United take on Bayern Munich from an executive lounge at Old Trafford.

What has happened since gives Hernández legitimate credentials to be recognised not just as a contender for bargain of the season but also one of the shrewdest pieces of Ferguson's transfer business in almost a quarter of a century at Old Trafford. Hernández sums it up as "hard to believe, especially when I was with my family when I got my medal and the Premier League trophy last Sunday".

As for whether the season can reach an even more exhilarating high against Barcelona, Hernández nods enthusiastically. "Why not? We know we are playing against 11 men, 11 very good men, but we are also a good team, of course."

He is waiting to discover whether Ferguson will play him alongside Wayne Rooney, as happened with such devastating effect in the first leg of the semi-final against Schalke. "Whatever happens," Hernández says, "it has still been a year I could never imagine."