Carlo Ancelotti has revealed that Roman Abramovich brutally threatened to sack him over a substitution he made in a Champions League quarter-final.
The veteran Italian boss - who is now the manager of the Brazil national team - spent two seasons in west London under the club's ruthless former Russian owner.
His first was a hugely successful one as Ancelotti helped the Blues win both the FA Cup and Premier League.
The 66-year-old's second was however less successful and ultimately ended in his dismissal at the end of the season after failing to win a trophy.
However in his new autobiography, 'The Dream - Winning the Champions League', Ancelotti has revealed that his Chelsea tenure could have ended a whole lot sooner when he substituted Fernando Torres in a European game.
Abramovich had splashed out £50m to bring the forward in from Liverpool and was more than displeased to see him sacrificed at half-time in the final eight of the Champions League.
Carlo Ancelotti enjoyed two years as manager of Chelsea between 2009 and 2011
He was working under Roman Abramovich, who ultimately dismissed the Italian after a trophy-less second campaign
The Russian had however threatened to sack Ancelotti months earlier over his decision to sub Fernando Torres in a Champions League quarter-final
Ancelotti revealed: 'Success or failure in Europe is how I would be measured by Abramovich, and the Champions League cost me my job.
'The night before the second leg (against United), Abramovich warned the squad that if they didn't win there would be changes.
'He then told me that if we lost, I should not bother coming back in the next day.
'Torres was clearly suffering from a lack of confidence and felt the weight of his transfer fee hanging around his neck.
'In the dressing room Abramovich made it very clear what this defeat meant. What happened to next was inevitable.
'Torres was his personal decision and substituting him was a direct rebuke to the owner. Momentarily, I had forgotten that ultimately you can't beat the owner.'
Ancelotti has worked for former Italian prime minister Silvio Berlusconi at AC Milan, Florentino Perez at Real Madrid and PSG head honcho Nasser Al-Khelaifi.
But he says none come close to the looming 'shadow' of Abramovich and the extreme pressure he felt his job was under during his time in west London.
Jose Mourinho (left) led his Inter Milan side to victory over former club Chelsea in a hugely damaging Champions League defeat in March, 2010
Ancelotti spoke in depth about working for Abramovich in his new book (above)
'I knew my record would get me only so far,' he added.
'Abramovich made clear he wanted me to win the Champions League with Chelsea and for Chelsea to carve out its identity on the pitch.
'But now I was working for a Russian oligarch who I suddenly understood expected everything to go well all the time.
'And if it didn't, he wanted to know why. It was my job to supply the answers.'
After an unexpected 3-1 defeat by Wigan, Ancelotti started to appreciate just how hands-on and demanding Abramovich could be.
'This was when the first hint of a shadow fell across my time at the club,' he said.
'Abramovich was at the training ground the morning after demanding answers. What had gone wrong?
'I never had this level of surveillance from Berlusconi. He was a demanding owner and sometimes bought players I didn't need and expected me to fit them into the side or argue about tactics.
'But for a lot of the time he was Prime Minister of Italy so there was no micromanagement. He had more important things to think about.'
And Ancelotti also revealed that Abramovich was furious after the man he'd sacked, Jose Mourinho, steered Inter Milan to a 3-1 aggregate win over Ancelotti's Blues in the Champions League.
Ancelotti said: 'The next day Abramovich didn't talk just to me but to the whole squad.
'My problem was that Mourinho's triumph was not great for my relationship with Abramovich.
'I was supposed to be the antidote to Mourinho - calm, measured and able to revive the squad after the drama.
'According to Abramovich, Mourinho was supposed to be a spent force.
Ancelotti was popular among the Chelsea players during his time at the club
Ancelotti says Abramovich had felt like Mourinho was a spent force when he left the Blues
'Allowing him to disrupt the script. I had embarrassed the owner. Success or failure in Europe is how I would be measured by Abramovich - and the Champions League cost me my job.'
A year after Ancelotti's departure, Chelsea went on to win the Champions League beating Bayern Munich on penalties in the final.
Abramovich's tenure came to a grinding halt when he was forced to sell the club to Todd Boehly and Clearlake three years ago.
Full proceeds from the £2.5bn sale remain frozen in a UK bank account and proceeds were supposed to be used to help victims of the war in Ukraine.
Earlier this year, a Foreign, Commonwealth & Development Office spokesperson said: 'This government is working hard to ensure the proceeds from the sale of Chelsea FC reach humanitarian causes in Ukraine as quickly as possible.
'The proceeds are currently frozen in a UK bank account while a new independent foundation is established to manage and distribute the money.
'UK officials continue to hold discussions with Mr Abramovich's representatives, experts and international partners, and we will double down on our efforts to reach a resolution.'