The smiling assassin: Why Juan Mata returns to Chelsea a player reborn

He will not be seeking to make personal capital out of playing against the man who considered him surplus to requirements at the soon to be champions. He will not stick two fingers in the direction of the Chelsea bench. Being Mata he is far more likely to greet the home manager with the warm embrace of old friends.

His polite demeanour, however, should not detract from the fact that Mata has been through some pretty challenging times over the past 18 months. Indeed, his emergence from a period of darkness is one of the most enriching stories of the season. It is proof nice guys can, despite all available evidence, come out on top.

Mata had been Chelsea’s player of the season for two years as the club won the Champions League and Europa League. Like his compatriots Santi Cazorla and David Silva he was the very epitome of the modern creative player as he fizzed around in Chelsea’s midfield. Not a conventional winger, but someone who preferred to move inside; not physically quick or strong, but in possession of a powerful brain; and with a habit of being in precisely the right place at the right time to score more than his fair share of goals: this was a pivotal part of the machine.

Mourinho, however, was suspicious of him. Almost from the moment he returned to the Bridge in the summer of 2013, the manager questioned Mata’s work rate. He wondered whether he would ever learn to track back. He could not work out how to accommodate him in a midfield of supreme ingenuity.

He did not believe Mata was equipped to play the game in the way he wanted Chelsea to play it. In the end, Mourinho decided that Oscar was a better bet in the number 10 position and he let the Spaniard go to Manchester United in January 2014. He did all right on the deal, too. United paid £37million for him. Which substantially helped Chelsea’s attempts to live with the Financial Fair Play rules.

The problem for Mata – and the reason Mourinho has not lost any sleep over transferring his most popular player - was that things were by no means resolved by the transfer. Despite his James Bond-like arrival in a helicopter, it was apparent the manager of his new club didn’t really want Mata either.

Bought by the chief executive Ed Woodward as a statement that he was, despite all appearances, a player in the transfer market, just as it hadn’t fitted into Mourinho’s, his style didn’t fit into David Moyes’s more leaden approach. The Scot couldn’t work out how to accommodate him, was nervous about ever playing him together with Wayne Rooney, Robin van Persie and Adnan Januzaj at the same time. For three months Mata must have wondered what on earth he had done wrong in a previous life to end up where he now was.

Even when Moyes was shown the exit at Old Trafford, the temporary management team appeared to be equally suspicious of Mata’s talent, like Mourinho not convinced of his contribution. And so it continued when the new manager came in. Mata did nothing wrong, continued to produce on the few occasions he was involved.

Yet under Louis van Gaal he was increasingly peripheral, stuck behind a misfiring Angel Di Maria in the pecking order, his appearances fleeting, his time largely spent watching from the bench. It looked as if his stay in the English game was dribbling to a terminal conclusion, his talent horribly squandered through no fault of his own, his stock sinking faster than Hull City.

But Van Gaal is nothing if not pragmatic. And when Di Maria was suspended for a laughably ill-disciplined sending off against Arsenal in the FA Cup, the Dutchman restored Mata to the starting line up. The reward for his trust was immediate. Together with Ander Herrera and Michael Carrick, Mata has been central to United’s recent revival. He is now simply undroppable.

His precision, his accuracy, the intelligence of his movement and distribution has helped bring out the best, too, in Ashley Young and Marouane Fellaini. Suddenly, as if by chance – though clearly Van Gaal would never admit to such a thing, insisting it was planned all along – United look a proper force. They look a team. Full of confidence, balance and belief in their own ability, they arrive at Stamford Bridge capable of slowing Chelsea’s inevitable advance to the title. Not derailing it entirely, but delaying it.

And, were he able to produce such a result, that would constitute the limit of Mata’s revenge. Like a train manager on Great Western, just being able to issue an apology for any inconvenience caused to Chelsea’s journey will be sufficient for him. That and a broad smile.