The main job of a manager is to motivate players. The best can pull off this trick year after year. When this ability fails then careers begin to drift. But what happens when a manager loses the knack of motivating himself?

Jose Mourinho looks tired. The man who arrived at Chelsea 12 years ago with a grin and an urge to cause mischief has gone, replaced by an older version who seems to have forgotten that football is fun. It does not appear to matter whether his Manchester United side are on the receiving end of a 4-0 thrashing at Chelsea, frustrating Anfield by shutting down Liverpool or trouncing the champions Leicester City 4-1. Win, lose or draw, the game seems to have become a joyless experience to the Special One.

The contrast with Jurgen Klopp after United's 0-0 draw was unmistakable. The German was disappointed but eager to discuss where his side went wrong -- his enthusiasm for the game was clear. Klopp's United counterpart, who, after all, had undergone a more successful 90 minutes, was sour and uncommunicative. Mourinho roused himself briefly to dispute the possession statistics, returning to the press room to declare that his own number-cruncher had counted United's time on the ball at 42 percent rather than the 35 percent figure most analysts used, but he was gray and distant compared to Klopp's bundle of energy.

The most exciting managers in the world are gathering in the Premier League and Mourinho seems to be a relic of a different age. Pep Guardiola, Antonio Conte, Mauricio Pochettino and Klopp live their club's games on the sideline, projecting vigour into their teams. Manchester City, Chelsea, Tottenham Hotspur and Liverpool have taken on the character of their managers. United have not shaken off the identity of the Louis van Gaal era. Mourinho projects himself in a way that's closer to the Dutchman's generation than the thrusting newcomers of the Premier League's new era.

At 53, Mourinho is not much older than Klopp, 49, and Conte, 47, yet it feels like he has many more miles on the odometer. The relentless politicking of his spell at Real Madrid and the unhappy remarriage with Chelsea wore him down. There is a popular theory within the game that the time at the Bernabeu "broke" Mourinho. Certainly, he returned to Stamford Bridge subdued, but winning the title with an unbalanced squad two seasons ago proved that this was still a manager to be reckoned with. The subsequent meltdown at Chelsea put Mourinho in a funk from which he still has not emerged.

During his first spell at Chelsea, the Special One's news conferences were the must-see event of the week. With his self-depreciating wit and charm, Mourinho enchanted rooms full of cynical journalists. Now the same writers queue up to see Pochettino, whose blossoming English has revealed a huge amount of humour and insight. Or Klopp, who has developed an even more distinctive character during his year in the Premier League.

When managers lose their focus, the players are the first to notice. It undermines the authority of the man in charge and once the grip loosens it is hard to get back. After Mourinho lost the Stamford Bridge dressing room he was in trouble. He is not anywhere near that troubling point at Old Trafford, but he needs to assert his authority and stamp his identity on the club.

Among the new wave of Premier League managers, Mourinho looks like a 1970s stadium rocker confronted by punk. He can still pack in the crowds and turn a tune when he steps on stage, but his shtick is tired. He has even taken to scolding the young guns for their overexuberance, rebuking Conte for his rabble-rousing of the crowd when Chelsea were 4-0 up at the weekend. Not so long ago, Mourinho was the rebel, the crowd-pleaser, the man everyone wanted to see.

It would be wrong to write Mourinho off, but he is no longer top billing in a Premier League brimming with exciting managerial talent. If he can find his old fire again it would be the greatest comeback of his career.