While the chasing pack are always keen to close the gap and attempt a coup, a champion's biggest challenge comes from within. And in the early weeks of Chelsea's horrific first third of the new campaign, it was obvious that this was not a challenge they were ready for.
Mourinho knows where the problems are in his team, is one of very few managers in the world whose track record suggests that he's got the skills to turn the current situation around and is stubborn enough to make the changes that are needed. If nothing's changed by, say, March, then the conversation becomes very different. But until then, as the kids say, #JoseIn.
Mourinho finds himself in perhaps his most testing time as a manager, but, more than ever, he needs his players to show exactly why they are champions and stop hiding from the cataclysmic disaster that is their current campaign.
The Premier League is highly regarded for its competitiveness, where anyone can genuinely beat anyone else on a given day - that kind of thing rarely happens in Spain and other European countries. There is no mystique or aura for defending English champions and it all means that when the naivety kicks in, as it has done in recent years, giants will always fall.
If it was naive to think that Chelsea were automatically favourites to win a second successive Premier League title ahead of the 2015/16 season - and it most certainly was - it's even more naive to think that now. Quite simply, Jose Mourinho's team have been left behind by their competitors and a terrible transfer window has now left them playing catch-up.
Chelsea manager Jose Mourinho is well known for being controversial throughout his career, but his latest outburst has left people baffled and annoyed...
Mourinho has failed to get the best out of Andriy Shevchenko and Fernando Torres before now. They, of course, weren't his own choice of players, but there is little evidence to suggest he will have any more success with Radamel Falcao.
With the quality players they have Chelsea had a good chance of beating United in a straight fight, but the players went out on the pitch prepped not to lose, rather than to try and win - the fact that they did get the three points was more a happy coincidence than anything else.
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Mourinho has the capacity to manipulate the media and to direct the press' attention where he sees fit. It is an attribute that so few top-flight managers possess but in modern football, it can be an essential tool in the coach's artillery.
Accustomed to watching their side roll over teams at home week after week, the City fans were again left in shock after a spirited Newcastle performance sent Pellegrini's men crashing out of the cup at only the second time of asking. There were no excuses for a crestfallen City.
Essentially, Mourinho is playing the Football Manager game with real people. It's a foolproof plan and the easiest way of turning up money to improve the first team squad... Chelsea's transfer strategy is long term and it's such a strong blueprint that it's going to keep them at the pinnacle of English football for the years to come.
The most telling moment this season at Anfield wasn't when he was dropped from the squad - registering just 34 minutes of football in 2015 at the time of writing - nor was it the time he was slammed by boss Brendan Rodgers for swapping shirts with Pepe at half-time. It was, typically, something that happened off the pitch.
Current league holders Manchester City should not be surprised if on January 31 they meet a Chelsea team that plays both freely because it is relieved of some pressure and angrily because it feels that the world is responsible for its defeat at the hands of Bradford in FA Cup.
Jose Mourinho and Chelsea couldn't get near it and as Thierry Henry once said, "Breaking the record and holding on to our invincibility - that's a hell of an achievement in English football, and nobody will ever be able to take that away from us".
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