He has changed countries, cities and clubs. He has changed coaches. Twice. He has changed culture and language. He has even changed his name. Or they have changed it for him. Johnny Kills has had a busy year. He won the European Under-21 Championships, joined Chelsea from Valencia for £23.5m, scored on his debut, beat Barcelona and won the FA Cup at Wembley. This week he was named the fans' player of the year and this summer he will play for the favourites at Euro 2012, looking to add to his World Cup winner's medal. Then he will compete at the Olympics. In his new home city. And before that, a European Cup final.

No wonder Juan Mata is beaming as he ducks out of the rain and into a café on the Kings Road. He left home at 15, swapping Oviedo for Real Madrid, and left Madrid for Valencia at 19, improving with every step, but even he didn't expect things to go quite this well. "This has," he says, "been perhaps the best 12 months of my career." So much for needing time. His initiation ceremony involved singing La Macarena in front of the squad and he admits finding cockney accents hard and Scottish ones near impossible, but to visit him in London is to find a man completely at ease. To watch him play is to see one too.

Daniel Sturridge's literal translation rendered him Johnny Kills. The name stuck. It fits too. "My adaptation on the pitch has gone in parallel with my adaptation to London," he says. Not for him the gated community in Surrey; there was no point living in London if he wasn't going to really live in London. Visits from friends and family have increased too: "More than ever came to see me in Valencia," he laughs. From the balcony of his flat, he looks out on the Thames. Battersea Power Station is within view – "Spectacular," Mata says, "the biggest brick-built building in Europe, a symbol of London, the Pink Floyd cover..." – and the city within reach. Mata pounds the streets and takes the tube.

"People just get on with life and I can be part of that normality," he says. "Sometimes they notice you but no pasa nada." Piccadilly Circus is 15 minutes on the underground. Concerts, galleries, musicals, theatres, restaurants. This week he spent his day off at the Tate Modern. He's seen the London Eye, Westminster, Trafalgar Square and more. So far, so standard. But he reels off some of the other places he's enjoyed: "Soho, the East End, Brick Lane, Spitalfields Market, Notting Hill, Portobello Road, Camden." Camden? "There's nothing like it in Spain: the atmosphere, vintage clothes, second hand record shops, books. Records for my uncles, fans of British music from the 70s. I call them from Camden or Kings Road or Carnaby Street and they talk Punk and Mods."

Mata has had to adapt on the pitch as well as off it and the transition has been as smooth, despite a change in management that might have worked against him. André Villas-Boas's sacking appeared to tilt the club's identity away from the Valencia playmaker brought to fit a new philosophy, and back towards Chelsea's former self. But from Spanish football to English and from Villas-Boas to Roberto Di Matteo, Mata has fitted in. In fact, he has stepped into a more central role. Any fear was immediately allayed by the new man in charge, of whom Mata speaks fondly.

A fan of English football, especially Manchester United's Ryan Giggs and Arsenal's Dennis Bergkamp, he admits that he doesn't remember Di Matteo as a player – "He says to me: 'Bah, you're too young!'" Mata says, smiling – but he has seen his coach's goal 40 seconds into the 1997 FA Cup final and that of his No2 Eddie Newton in the same game. "Robbie said he was lucky," Mata adds, "but it was a golazo."

"English football is changing," he continues, "the champions don't play a 'typical' English style, for example. But in general it's quicker than in Spain: more counter-attacks, more open, more direct. André Villas-Boas was a little more European, trying to bring the ball out and build from the back. He did everything he could to make it work but it just didn't happen. And everything depends on results.

"Roman [Abramovich] came to the training ground and he told us: we're Chelsea, we need a change. When Robbie came in the priority was to win come what may. If that means being more direct, fine. We were on the edge. The psychology is very important: we won the first game in the FA Cup with Robbie and then the next and the next ... that changed everything.

"We've played lovely football – against Spurs for example – and also competed. We can do both. We've got players who won lots here with high-intensity football and others who are more technical, like Lampard or Meireles.

"André was an important reason why I came. But from the first day Robbie took over, he gave me confidence. He told me it was a difficult moment because of the departure of André, who is a friend of his, and that we had to look ahead. He told me I was an important player. We've changed the system. Instead of a 4-3-3 we play a 4-2-3-1, with me in the centre."

Slowly, Chelsea set about rescuing their season. Opportunities arose. First in the FA Cup, a competition treated with reverence by the Spanish and the moment that has most marked Mata, and then the Champions League, one that started back in the autumn with Mata returning to Valencia to face his former team. Then Napoli, Benfica, Barcelona.

"When you play against Barcelona, you talk to the coach about how you can compete. You reach the conclusion that there is a way you can beat them — if you are effective in taking your chances, as we were. We didn't simulate the game in training; there was no 'this is Xavi', 'this is Messi' kind of session but we watched videos and we talked a lot. The coach told me my job would be to play on the right, close off the wing and generate counter-attacks, looking for Didier Drogba, Fernando Torres, Ramires — the fast players.

"I like to be in contact with the ball, have possession, to play. But we knew that it was going to be a different type of game. Maybe not as pretty but that's what we had to do. And that's what makes you reach the final."

Did you enjoy it? "During the game, I suffered. You run and you run and you don't touch the ball and you defend. You suffer. [When Terry was sent off] you think it's going to be very, very hard. A man down, losing. But Ramires's goal gave us life. At half-time, the manager told us we'd done that hardest part: score. If Ramires hadn't scored and in the moment he scored, I think we wouldn't have got through. Half-time, 2-0 down, a man less. Barcelona would have grown. But that goal changed everything. There's a reason it was voted Goal of the Season!

"We still suffered though. After I came off, sitting on the bench, I didn't want to look. I had a terrible time. I was sure they were going to beat us. You see the chances – the penalty, the post, they're getting closer, and you think: 'They're going to score, we're going out.' It was horrible. I was nervous, alongside Gary the kit-man. When Fernando scored, we just exploded. At the final whistle, I ran straight for Fernando because he deserved it: he'd seen us to the final after everything that had been said."

And so to the final, where Chelsea – again – will be the underdogs. Bayern Munich played superbly against Real Madrid and are at home. The balance of suspensions is tilted slightly in their favour too. "They're missing the left-back, one in the middle, a central defender; we're missing Meireles, Ramires, Ivanovic, Terry – four starters in Barcelona," Mata says. "If it's not a really bad foul, an aggression, something that deserves a direct red, maybe there needs to be a different punishment. It's the biggest game of a player's life.

"Bayern being favourites could be good. And we're treating this like a normal game. We've stayed in London until Friday and been working at Cobham as usual. Of course there are always two readings: it might feel too 'normal' for a Champions League final or it might be that by doing that we take the tension out. I think it's been the right decision.

"For those who played [in 2008] you can see what it means to them to pull that thorn out from their side. You never know if you will get another chance – and not just the more experienced players. Lampard and Terry are legends here. For everyone at the club, for Roman, the players, it's important to have the European Cup in the cabinet. There still isn't one. This game is different."

"There's always pressure. People's happiness depends on you; they suffer with you. You get used to it but you have to know how to handle it. I wouldn't call it superstition but routine helps, always doing the same thing. You eat, watch videos, meet up in someone's room, chat. A stroll on the morning of the game. And I talk to my mum on the way. 'I'm going to the ground', 'good luck', 'see you later'. It's just a quick call, but I always make that call.

"We were sixth in the league: we know Chelsea have to finish higher. But we've won the FA Cup. At times it's been difficult, with a change of coach and some bad moments, but it's been a good year. It could end up being a magnificent year."