London calling. But this time Thibaut Courtois is not picking up. Never mind the £9m transfer, the text-message exchanges with Petr Cech, the friendship with Eden Hazard, the calls and the visits, never mind the trips back to Cobham, or even the five-year contract at Stamford Bridge – Courtois is the Chelsea goalkeeper who wants Chelsea to lose. For now. No mixed emotions, no doubts, just a simple: "Sorry". That's just the way it is.

No one expected it to work out this way. He was supposed to be playing; he was not supposed to be playing against them. Courtois joined Chelsea from Racing Genk as a 19-year-old last summer and was immediately sent out to Atlético Madrid on loan.

At the end of a hugely impressive first season, Atlético won the Europa League. At the same time, Courtois's other team were winning the Champions League. But for a trip to Colombia, he would have been at the Allianz Arena to watch them, part of the official expedition. Instead, he had to watch it on television.

They face each other in Monaco on Friday – Europa League winners versus Champions League winners; the club Courtois plays for against the club that owns him, the one he will play for one day.

Courtois says there have been no calls. No suggestions that he let a few soft ones through his legs? He laughs. "Nah. The last [Chelsea] person I spoke to was Eden Hazard when we were in the national team and I said: 'Yeah, we're going to win.' He said: 'Oh, I don't know.' At that moment Chelsea were in pre-season, they were not playing that good, but now they've won three times in a row with good football, a lot of goals, and we have to manage that.

"I'm like [Fernando] Torres," he says in virtually flawless English. "It will be special for him because he played here, but if he has to be honest with himself, he wants to win. If Atlético win, he can't say: 'Oh, I won a bit too.' I'm the same. If Chelsea win, yeah, OK, maybe I am a little bit happy for them, but I will be disappointed. When I go in maybe one or two or three years to Chelsea, yeah, I will want to win the Champions League with them and the Super Copa with them, but now I am at Atlético Madrid.

"Chelsea is my team, but, because I have never really trained with them or played a game, I cannot feel like it is really my team. When they won the Champions League I was really happy, but it was not like I won."

People say: 'Yeah, you won the Europa League and the Champions League in the same season!' I say: 'No, I won the Europa League, but not the Champions League.' If you are a player who never plays, it's the same. You can say you won the Champions League if you are on the field or if you had minutes, but not if you didn't.

"Chelsea is my team, but Atlético is my team now."

Courtois comes from a family of Belgian volleyball players. Both his parents played professionally and his brother does, too. He could have followed them. Standing at 6ft 6in and agile, the inheritance is clear. "My father played football until he was eight and then he changed to volleyball," said the goalkeeper.

"All my family are volleyball players and when I was young I liked to play beach volleyball. I like to be in the sand, to jump. I played football too, left-back, and when I was are eight or nine years old, they asked: 'Do you want play in the goal?' and I said 'Yes, it's good.'

"Maybe if I had decided I wanted to be a left-back, I might be in the third division in Belgium, and now I am here, so it was a good choice. I was at the team where I grew up for 11 years, but I only played one game in the first team when I was 16. Then for three years, nothing. When I was 18, I got into the first team and played half of the season. We were champions and I was the best goalkeeper in the league. Chelsea signed me and I came here on loan. Last year was only my second season as a first team [professional]."

He adds: "It has gone very quickly. It is strange for a goalkeeper of 20 to win the Belgian league – OK, maybe it is not as strong [as England or Spain], but for me it was nice to win the league with a team that is not normally playing to be champions. And then the next year, the Europa League. And now I am in the Super Cup. So in three years I can get three trophies, maybe."

That season in Genk was impressive enough for Chelsea to spend £9m on Courtois. But at only 19, and with Petr Cech still going strong, they wanted him to progress and develop. Atlético were not seen as just somewhere to go, but as an apprenticeship.

Chelsea have accompanied his progress, laying down training regimes, monitoring his performances, providing advice, periodically bringing him back to London. But just as they wanted him to go to Atlético, they wanted him to leave again. It was time for the next stage. This summer, the idea was for Courtois to go to another English club. He resisted. And eventually he got his way.

Spain has been good to him. He lives in Boadilla del Monte, a 10-minute drive from Atlético Cerro de Espinatico Cerro de Espina training ground to the west of Madrid and he has picked up Spanish fast. Colloquial Spanish too. Every day, staff say, he comes in with another phrase, picked up from adverts or TV. They say he is a cachondo, a joker.

Courtois's father is from the French part of Belgium, his mother the Flemish. And as well as Flemish and French he already spoke English. "Everything on television in Belgium is in English with subtitles," he explains. He even spoke a bit of German. Now his Spanish is almost fluent. And without taking a single lesson.

"With French it's easy," Courtois says. "Mind you, I look at my father and he can't speak a word of Spanish! I have been listening to my team-mates. They would say to me: 'No, Thibaut, it's like this or like this.' There were lots of jokes. It was good to learn that quick without lessons. Now I can go to the store, I can buy what I want. If I have a problem with my mobile phone, I can call and fix it.

"And there's the swearwords, of course. In Belgium, I already knew puta madre, but when I came here it was the first thing they taught me. It was funny for them. They would say: 'Ah, Thibaut, go and say me cago en tu leche [I shit on your milk] to that player,' and everybody laughs. It's also good for my integration."

That was a key part of Courtois's thinking when Chelsea proposed a move back to the Premier League this summer. There were footballing reasons, too. "There was a chance that Chelsea says: 'Yeah, Thibaut, Atlético is good but we want you back in England; we want you to play at this team or this team.' But they know also that I am very happy here and that I have learnt a lot.

"[Chelsea's goalkeeping coach] Christophe Lollichon watched me a lot, so he saw that it's a good league and that I am playing well. They would rather have a happy goalkeeper playing in a good team in Spain than playing for a team fighting relegation in England. OK, I would adapt to football in England if I went there on loan, but you will let in two or four goals every game and that is not good for your confidence. For that reason, I think it was easy for them to say: 'OK, you can stay there.'"

He adds: "There isn't really a [road] map, a plan for the future. First they [Chelsea] had a plan to loan me maybe one, two, years, then maybe two years somewhere else in England. But it also depends how well I play, how well Cech plays. Can he still play? I cannot say that I will come back to Chelsea next year. We will have to see in April or May. Maybe they say: 'We want you back at Chelsea.' But maybe they will loan me to another team. You shouldn't forget that I am only 20 years old and there is time to learn. At the moment I don't know. But the relationship is close."

Courtois exchanges texts with Cech and they have met at Cobham, as well. Meanwhile, Lollichon is keeping a close eye on him, guiding him through his development. "The goalkeeper trainer came three times to see me in Spain, he came to see the national team, he came to the [Europa League] final in Bucharest," Courtois says.

"He asks me about training, he spoke to the [Atlético] goalkeeper coach to see how we worked here and I went back to Chelsea to do some physical tests. They have given me [a programme]: I had to do some fitness work here, some gym work to get some muscles. Well, not muscles, but build strength because it's true that English football is more physical, that players go into the keeper and the referees don't whistle.

"Chelsea don't want to say: 'Go to Atlético and we don't hear from you,' because they know I am a promising goalkeeper and so they keep following me. Maybe if you are a 26- or a 28-year-old player and you don't have a future and you can't be sold, they send you on loan and they say: 'You go there and leave us alone.' But it's not the same with me."

Courtois comes full circle. Again, he notes, he still hasn't played for Chelsea. He is a Chelsea player, except that he isn't. Watching last year's Champions League final was, he says, "strange".

"I was happy that Chelsea won. In Spain, there was a lot of reaction for the way they beat Barcelona, but I said: 'Yeah, what do you want?' Everyone knows how strong Barcelona are and you have to find a way to beat them and Chelsea defeated them. It doesn't count how you play, what counts is the result.

"Maybe we play a very bad game against Chelsea [on Friday] and still we will win. So maybe people will say: 'Chelsea should have won,' but if we win, that's what matters. And that's what I want."