Tom Adams: United need to be wary, Guardiola brings out the worst in Mourinho

The fear that Jose Mourinho is not the right manager for Manchester United is not a hypothesis without supporting evidence. Nor does it have to be inferred from the kind of leaks that are becoming business as usual at Old Trafford. In fact, it has already been publicly endorsed by one of the most beloved United figures of all and a current director.

"A United manager wouldn't do that," said Sir Bobby Charlton in 2012, in response to Mourinho's behaviour on the touchline in the Spanish Super Cup a year previously, when he plunged his finger into the eye of Barcelona assistant Tito Vilanova. "Mourinho is a really good coach but that's as far as I would go really."

It is a four-year-old appraisal now, and one which has been widely cited in discussions of Mourinho and United ever since. Sir Bobby may well have changed his mind in the intervening years, though the nature of Mourinho's implosion at Chelsea this season might equally have reinforced his concerns. It is intriguing to wonder whether Sir Alex Ferguson, a regular presence at Old Trafford, still feels the same as he once did. "He doesn't like him too much," said Sir Bobby.

The point is that this rare insight into the minds of those who wield influence, if not outright power, at United has never been more relevant. Not only is Mourinho being handed control of Old Trafford, his arrival in Manchester will put him in direct competition with the man whose dominance of La Liga coaxed out his worst characteristics: Pep Guardiola.

The renewal of the bitter rivalry between two former colleagues at Camp Nou is a compelling story and it is worth revisiting how just toxic it became between 2010-2012, when the greatest rivalry in European football became consumed by a mortal battle between the two most famous coaches on the planet.

Journalist Diego Torres chronicled this fascinating period in his book, 'The Special One: The Secret World of Jose Mourinho' – which, it must be noted, has been rubbished by Mourinho's camp. Torres painted a picture of a paranoid control freak whose all-consuming mission to bring down Barcelona developed into a kind of total warfare, where Mourinho used every weapon available to him to try and destabilise Barca and their coach, even going as far as to instruct his players to complain about supposed bias in the fixture list during interviews.

Mourinho's war of attrition, played out in the media, finally erupted in open conflict in April 2011 when, prior to a Champions League semi-final between the two Spanish giants, Guardiola snapped.

Having been accused by Mourinho of criticising referees even when their decision were correct, a bristling Guardiola told a press conference: "As señor Mourinho has called me Pep, I'm going to call him Jose. Tomorrow at 8.45pm we will face each other on the pitch. Off the pitch he's won. He's been winning off the pitch all season. Let them give him a Champions League for it so he can enjoy it and take it home. In the press room he is 'el puto jefe' (the f***ing boss) and the one who knows more than everyone else."

Guardiola's players gave him a standing ovation when he returned from his media duties and after knocking out Madrid they went on to add the Champions League to the Liga trophy they won that season.

In 2012, though, Mourinho did topple Barcelona, winning the league with more than 100 points. And this despite persistent dressing room unrest in response to his methods - according to Torres at least.

The Torres book can be read in part as a meditation on whether winning is everything, the end that justifies any means, or whether style - as expressed both on and off the pitch - matters. From it you get the impression that the more established members of the Real Madrid team, bred on regal tales of Madrid's glorious history, believed results were not the only consideration. It is an issue you imagine Charlton has come back to frequently over the past four years.

So why is this relevant now? Because the Mourinho/Guardiola feud has continued as a Cold War which will be reignited in Manchester next season. Even as he secured the Premier League title with Chelsea in 2015, Mourinho was aiming a dig at his old friend and then rival, also a league winner with Bayern Munich. "Maybe in the future I have to be smarter and choose another club in another country where everybody is champion,” he said. “Maybe I will go to a country where a kitman can be coach and win the title."

Perhaps it is too easy to draw a dividing line between the charismatic and bullish Mourinho of Porto, Chelsea and Inter and the more antagonistic, uncompromising figure of the Mourinho of Real Madrid and his second spell at Chelsea and conclude that it was the quest to destroy Barcelona which changed something in him. It is not as if Mourinho hasn't always had a bit of devilry about him. But the Torres book does paint a convincing picture of a malign and destructive influence rather than a Special One feted for his charismatic management. The Guardiola rivalry clearly brought something out in Mourinho - and Sir Bobby Charlton saw it.

At least the two men were separated by 380 miles in Spain; two cities, two cultures and two regions keeping their dispute at a distance. Next season, they could be near neighbours in an exclusive and cloistered millionaires’ row somewhere in leafy Cheshire. Even as they unveil a man who can restore them to the very top of the game, United must be concerned on some deeper level that Charlton’s worst fears will be realised.