When Jose Mourinho arrived in London in 2004, he declared himself the Special One.

Maybe, having won the UEFA Cup and Champions League in consecutive seasons with Porto, he was entitled to his bravado, but arriving in the capital city meant encroaching on another man’s turf.

Arsene Wenger was special, too. In the summer of 2004, his Arsenal side were still Invincible. Indeed, it would take another ten Premier League games of the 2004/05 season until they were no longer the league’s unbeaten side.

But Wenger’s side found it impossible to keep up with the pace they set for themselves the season previous, and succumbed to five defeats. That year, the best team in the league was Chelsea, and Mourinho proved that his bite matched his bark as his side ended the season top of the table, with only one defeat and only 15 goals conceded. They were indeed special, even if that solitary defeat away to Manchester City is what prevented the Portuguese coach from being able to claim for himself what Wenger had done the previous season and have his own side spoken of in the same vein.

It didn’t matter, though, Wenger’s season wearing the Premier League crown was over. And his time as the most innovative manger in the league was gone, too.

That’s because things changed that summer in 2004. When Mourinho arrived, it wasn’t just that a new manager had rocked up in town. It couldn’t even be explained by the fact that a billionaire owner in Roman Abramovich had bought Chelsea and bought – or attempted to buy – a league title. It was more than that. Chelsea couldn’t ‘buy’ the title the year before when Claudio Ranieri’s side came second to Wenger’s Invincibles. But Mourinho didn’t just take Chelsea up one rung in the ladder. He changed things permanently.

Arsene Wenger’s downfall came in stages. After an FA Cup triumph in 2005, and a Champions League final appearance in 2006, it became more and more pronounced with each passing season. Stadium repayments, the arrival of more and more moneyed teams into the elite, and an ever-changing game put Wenger out of touch with success. But it was Mourinho who sped things up.

When he arrived, the former Porto coach set about changing English football by adding an extra man in the midfield. Claude Makelele wasn’t always considered a midfield spoiler, but that’s the role he came to be known for, as Mourinho’s pragmatic view of the game took root. Like all revolutionary ideas that take hold then take over, that one was a very simple idea executed to perfection.

There’s more to it than this, of course. The players Chelsea had and the ideas the manager had can’t simply be boiled down to an extra man playing a new position. But this, above all else, seems to have been the first nail in Wenger’s coffin.

For the next decade and more, the Arsenal boss has been considered tactically inflexible and unable to move with the times. He was the future once, but the accusation levelled at him is that he is no longer even the present now. And it’s the introduction of a three-man midfield that seemed to render his approach outdated.

Wenger has always played a very specific way as Arsenal boss. There were games early on where he’d line up with three centre-backs, and similarly again this season. But for the rest of the 20 years or so in between, there was one system: 4-4-2. Or, in modern parlance, we may now tweak it and call it 4-2-3-1.

Two centre backs, two full backs, two defensive midfielders, two wingers, and a number 10 playing behind the striker. Some variation on that theme was Wenger’s staple. Whether it was Dennis Bergkamp behind Thierry Henry and in front of Patrick Vieira and Gilberto Silva, or even Mesut Ozil behind Alexis Sanchez, and in front of Granit Xhaka and Francis Coquelin, it hasn’t changed much in two decades.

That works incredibly well against traditional 4-4-2s. The defensive midfielders drag the opposition central midfielders towards them, leaving space for Bergkamp or Ozil to roam in. Meanwhile, the opposition defence is worried about the pace of Henry or Sanchez, and drops deep, creating even more space for the maestro.

The problem is, when you throw a defensive midfielder like Makelele into the mix, that space is limited, and your maestro is much less effective. And with the success that Mourinho had with an extra man in the ‘Makelele role’, everyone started to do it, and Wenger’s success dwindled. Since Mourinho arrived in England for the first time, Wenger hasn’t won the league. And until the 2015 Community Shield, the Frenchman hadn’t ever beaten the Portuguese. That’s not just an interesting piece of trivia. It’s no coincidence.

Nor is it the whole story, of course. It’s not the only reason why Arsenal haven’t won a league title, and it’s not the only reason Wenger has failed so often in the second half of his tenure. Writing about all of those reasons would fill books. But it is a significant turning point along the way, and an easy-to-pinpoint moment when the Invincible manager at the top of the world was reduced to a man with only three FA Cups to his name in 13 years. A man whose managerial exploits will always earn him a place in the history books, but whose position – right down even to his role at the club – looks more like an historical oddity than anything else.

The rise of Chelsea, beginning with Roman Abramovich, but most notably with the arrival of Jose Mourinho, changed the balance of power in London away from the north and towards the west. But it was the spread of Mourinho’s ideas all across the country and Wenger’s inability to adapt that caused the Frenchman a decade of toil.

When Mourinho arrived, it was Arsene Wenger who had the most convincing case to anoint himself as the ‘Special One’. In the opposity way, the same is true today.