Diego Costa was standing in the Sunderland penalty area at Stamford Bridge when he looked over to the touchline and saw the No 19 flash up on the digital board to signal that he was being substituted for Loic Remy.

There were 15 minutes of the game left. He knew it was going to be a long walk.

He walked slowly anyway. The boos from the Chelsea fans, his own fans, started after he had taken a few steps. They were not deafening by any means but they were loud enough.

Diego Costa was booed by sections of Stamford Bridge support when he was substituted against Sunderland

Some Chelsea fans vented their anger at some players after the sacking of Jose Mourinho on Thursday

A collection of Chelsea fans turned on their out of form squad in support of their axed manager Jose Mourinho

A section of the home crowd unveil a 'you let us down' banner as the Chelsea fans support their former leader

Costa glared around at the supporters defiantly, staring them down.

As he made it to the touchline, the entire stadium had started chanting the name of Jose Mourinho as if to reproach him with the name of the man they thought he had betrayed. Costa remained defiant.

He made a point of high-fiving every single member of the Chelsea bench.

‘Jose Mourinho,’ the crowd kept singing. ‘Jose Mourinho.’

This particularly scathing banner attacking the playing squad was on show at Stamford Bridge on Saturday

Up in Manchester, they were singing Mourinho’s name at Old Trafford, too, although it was the Norwich City fans who were responsible for that, making sure that they added to Louis van Gaal’s discomfort.

Their mischief was well aimed.

United supporters’ discontent with Van Gaal scaled new heights with the 2-1 defeat by Alex Neil’s side and pulses in the north have been quickened by the knowledge that Mourinho is ready to go straight back to work.

A cynic might say that the statement released on his behalf on Saturday morning was aimed at the United hierarchy and United’s fans to make doubly sure they know he’s available.

Manchester United fans were upset with manager Louis van Gaal after a 2-1 defeat by Norwich at Old Trafford

Mourinho was on the south coast on Saturday, watching Brighton against Middlesbrough but it felt like he was everywhere.

The cult of personality that surrounds him appears to have survived the fact that he left Chelsea one point off the relegation zone with nearly half the season gone and there is now fevered speculation that United, who slid out of the top four, will hit the panic button, dispense with Van Gaal and install the Portuguese in his place.

These are strange times in the Premier League.

The axed Chelsea boss Mourinho was in the stands at Brighton watching their loss to Middlesbrough

Mourinho remains popular at Old Trafford and would be in contention if Van Gaal's poor run continues

There is a frenetic, anarchic feel about this season that was underlined when Leicester moved five points clear at the top of the table with their victory over Everton at Goodison Park.

English football’s superpowers are struggling to keep it together. There is a sense that amid the mediocrity of much of the football, huge shifts are taking place.

Part of the reason for the mourning for Mourinho at The Bridge was the realisation that this, finally, is the end of an era.

His previous absence from the club, between 2007 and 2013, felt like an interregnum. Even when Chelsea won the league and the Champions League without him, it still felt like it was Mourinho’s team. Mourinho still felt it was his team, too.

Jose Mourinho's latest departure from Chelsea feels like the end of an era at the English champions

New Chelsea boss Guus Hiddink watched Chelsea's 3-1 win alongside Didier Drogba and Roman Abramovich

This time, though, it’s over. There will be no going back. Guus Hiddink may only be a temporary replacement but his successor will not be Mourinho.

He is now the club’s glorious yesterday, someone who will always be idolised here for what he achieved.

It would hardly be a surprise if United leapt into his arms. Van Gaal, increasingly, is starting to look as if he, too, has lost the players and United’s decision-making dynamic is bound to be affected by the strong rumours that Pep Guardiola has decided to leave Bayern Munich at the end of this season and will join Manchester City.

When Sir Alex Ferguson retired as United boss, it was widely felt that Mourinho would not be right for the philosophy of the club. United were empire-builders then.

United vice-chairman Ed Woodward would jump at a trophy signing like Mourinho to buy himself time

They craved the solidity and the longevity of another Ferguson, someone to build a new dynasty.

There was opposition to the shock-jock philosophy of Mourinho.

The grand old men of the club, people like Sir Bobby Charlton, distrusted the idea that Mourinho would come in and win trophies but traumatise the club in the process and leave after a maximum of three years. Mourinho was passed over.

But now things have changed. Sir Bobby’s influence has waned and United’s executive vice-chairman, Ed Woodward, is a man who would jump at a trophy signing like Mourinho to buy himself a bit more time after the mess he has overseen since he succeeded the former chief executive, David Gill.

Sir Bobby Charlton distrusted the idea of handing the Portuguese the reins at Old Trafford

The Manchester United legend's influence at the club has waned since the appointment of David Moyes in 2013

Woodward likes big names. He does not give the impression of thinking long-term. To him, long-term is signing a three-year deal with a new noodle partner.

He wants glitz. He wants results and he wants them now. He is more about style than substance.

Mourinho would jump at the chance. There would be no curbs on his spending at Old Trafford, no worries about Financial Fair Play.

If Woodward and the Glazer family have lost patience with Van Gaal, Mourinho may also find he has an ally in Ferguson, who has long been an admirer and friend of the Portuguese.

Pep Guardiola has long been coveted as the Bayern Munich coach prepares to make a decision on his future

There are growing fears at Old Trafford that, despite the hundreds of millions of pounds Woodward has thrown at new players like a kid splattering paint on the wall, United are getting left behind.

The best young players are going to City’s academy and United’s transfer policy is being undermined by Woodward’s inexperience. If City get Guardiola, then the gap is likely to increase.

There are signs that even Van Gaal may be starting to doubt himself.

‘I am, or was, a very successful manager,’ he said last night.

Maybe it’s time to call for Jose, a man who has been allowed to brush off his failure at Chelsea as if it were an impertinent impostor, a man for whom self-doubt remains an exotic stranger.