For Thibaut Courtois, the true rite of passage came in his initial training session with the senior Belgium squad. The teenager had been called up for critical Euro 2012 qualifiers against Kazakhstan and Germany, a rookie of huge promise but an Atlético Madrid loanee still digesting a thrashing, the most conclusive of his fledgling career, inflicted by Barcelona a fortnight before.

On a muddy pitch in Woluwe, Georges Leekens split the squad into attack versus defence, ordered the first-choice keeper, Simon Mignolet, to wait his turn behind the netting and chivvied the youngster to marshall his back line. “Vincent Kompany and [Daniel] Van Buyten were there, two big players, and I was 19 and in the squad for the first time,” he says. “As a young guy, I was afraid to shout at them when we were organising ourselves for corners or set plays, to tell them who they had to mark or where to stand. You don’t dare. But, even if you can be shy at first, you have to learn how to make yourself heard.”

Courtois’s performances in the three years since have screamed pure pedigree. As a gauge of how far he has come, fast forward to last week’s friendly against Iceland and the exasperated critique, his frustration plain at what he perceived to have been sloppiness from victorious team-mates. Some in their number had neglected defensive duties and, he told the manager, Marc Wilmots, should have been replaced. A fuller explanation was later offered once the adrenaline had subsided, but the outburst had come from a 22-year-old who has won domestic titles in Belgium and Spain, claimed major silverware in all of his campaigns as a professional and is now impressing with Chelsea, the Premier League leaders. It demanded recognition.

In summoning Courtois from what had felt like a perma-loan in the Spanish capital, Chelsea offered arguably the most promising goalkeeper in the world his opportunity to excel. The focus has fixed on more high-profile recruits, drawn to Diego Costa’s brawn and bite, or Cesc Fàbregas’s blend of invention and cleverly disguised needle, but the Belgian at the back has busied himself maintaining his brilliance from La Liga. At 6ft 6in, he is imposing, a formidable presence despite a slender frame. He has diligently blocked or palmed away much flung at him during the club’s unbeaten start to the season. John Terry and Gary Cahill already have faith in the new first-choice. The biggest tribute to the positive impact he has made is that Petr Cech, a club stalwart and consummate professional who had done nothing to justify his omission, has not really been missed. This has been a changing of the guard.

It was José Mourinho who had been so determined to curtail the cycle of one-year loan arrangements in Madrid. Courtois had arrived from Genk in 2011, undertaking his medical on the day the first-team departed for a pre-season tour of the Far East, but he was subsequently schooled – with regular input from the Chelsea goalkeeping coach, Christophe Lollichon, and even visits from the long-standing back-up to the back-up, Hilário, to check on progress – at Atlético.“There was no plan set in stone when I arrived, but it was about playing in the future, and dependent upon development,” he says. “The first two years at Atlético had gone well and, in 2013, Chelsea had wanted me back but I wasn’t 100% sure. I knew a third year in Spain would maybe be a chance to do something special. We did that by winning La Liga and reaching the Champions League final.

“After that, it felt the right moment to go to Chelsea. The manager had called me just once, earlier this year, to give his impression about the season ahead. He was talking about this campaign, saying how he saw things for all the team. He thought we’d have a strong side and wanted me to be a part of that. That was the gist: he hoped I’d come back.

“Working for him now, it is easy to see why his sides have that fighting mentality. He motivates you. If the coach on the sidelines is not that strong, transmitting the kind of spirit he has, players won’t always handle it well. Life becomes difficult. But, with Mourinho, everything is good. He knows when to be among his players, as ‘one of us’ making jokes as a friend, and when to be strong and distant, even severe. That’s how it has to be to get the team sharp. [Diego] Simeone did that, also, in his own way. He was an ex-player so he knew how we ticked.”

The Argentinian, who inherited Courtois midway through that first season in Spanish football, twice masterminded victories over Chelsea with the loanee in his side. The Uefa Super Cup trouncing of Roberto di Matteo’s team had signalled the beginning of the end for a Champions League-winning manager. The semi-final defeat in that competition in April effectively ensured Filipe Luís, Diego Costa and Courtois would all become members of a Chelsea revamped lineup.

That 3-1 reverse at Stamford Bridge on 30 April is the last time Chelsea were beaten. “Not a lot of teams could have beaten us at that moment,” says the goalkeeper, whose smart save from a Terry header had helped smooth progress. “We had our organisation. That was one of our biggest qualities, as well as individual class from Koke or Costa. Maybe, individually, we were not the best in Spain. If you look at us individually, this team at Chelsea is stronger than that side at Atlético last season. But what was important was to work hard for each other and to be selfless: a unit; run for one another; recover one another’s mistakes; give everything you can.

“This side is also doing that, which is maybe why we’re top of the league. Not only do we have the individuals, but we also have the team ethic. That fighting mentality. At Liverpool we trailed but won and that’s not easy at a place like that. Other games, when we’ve been struggling, we kept on playing and always came out with something.”

West Bromwich Albion arrive on Saturday seeking to check the hosts’ momentum, though a four-point gap to Southampton, and eight to the champions, Manchester City, tells its own story.

Courtois signed a new five-year contract in September and has been unflustered through much of the campaign with his one real moment of anxiety, a collision with Arsenal’s Alexis Sánchez, which left him dazed and taken to hospital, serving to illustrate the depth of his acceptance in new surroundings. Cech replaced him on the field but, even granted a potential route back into the team, the older man had other concerns. “Not long after the game had finished, and I was in hospital, Petr tried to call me twice and later he texted me,” says Courtois on the day the winners of the joint Guardian, Audi and Chelsea competition received their prizes.

The injury was less serious than initially feared but Cech, who had suffered a depressed fracture to his skull eight years ago, spent the evening advising the man who has usurped him how to use his shoulder and back to protect his head from an impact.

The pair are competitors but friends. “On the day I took my original medical here, Petr came to say hello and welcome me, just like Drogba and John [Terry]. They made the effort. We train really well together now, talking a lot about things that can happen in a game. It’s a nice relationship. I understand that, maybe, he’s not happy he doesn’t play a lot and that’s normal. I would be the same. But we get on well. At Atlético, the No2s would not always speak to me. It wasn’t jealousy, more frustration, but it meant we didn’t get on well. That’s a pity because, normally, the goalkeepers are just alone working together. It’s not like that here.

“I’d watch Edwin van der Sar and try and steal things with my eyes from how he played, because he wasn’t all muscles and was a similar build to me. But Petr was another I’d look up to, someone I wanted to emulate.”

Cech was also a member of a core of senior players whose influence at Chelsea was clear. Drogba, Terry and Frank Lampard were also counted in its number, elite veterans of Mourinho’s first spell in charge or before. A new team has developed since, a side that leans as much on the flair of Eden Hazard or Oscar, the industry of Branislav Ivanovic and, steadily, the security offered by Courtois at their back. He recognises he must be even more vocal with his back line and confront the physical nature of the division. But, just as he grew within the Belgium setup, now he boasts the clout to make himself heard, if required, at his club.

“In the national side Kompany is the real leader but he was not there [against Iceland]. I don’t feel responsible for all the team, but I am responsible for protecting my goal. With what I’ve already achieved in my career – winning trophies and playing in finals, important matches against Real Madrid and Barcelona, winning the Europa League and the Super Cup, and in the Champions League – sometimes you’ve earned the right to say something.

“I’m not saying I’m the leader, but I feel I can say something if it will improve the team. At Chelsea we have JT, a great leader, as well as players like Drogba. In the dressing room I’ll leave the shouting to them. Maybe that will change in the future but, for now, I’m just new at this club.”

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