Mourinho was hired by Madrid in the summer of 2010 to knock Barça off their perch - and he would achieve that by wearing down the Catalan giants’ manager, his former friend, until he was forced to walk away from the job two years later.
Now reunited in England, the two coaches are ready to go head-to-head once again this weekend as the first Manchester derby of the season takes place on Saturday lunchtime.
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As well as each other, Mourinho and Guardiola have fallen out with quite a few high-profile players during their respective managerial careers.
Scroll down to see our list of players who have either fallen out with Mourinho, Guardiola, or even both.
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Five stars who fell out with Jose Mourinho
Mourinho and Casillas fell out big time during their final year together at Real Madrid.
The Portuguese coach dropped the Real Madrid legend in favour of Diego Lopez because, in his view, the former Villarreal and Sevilla player was a better goalkeeper.
However, according to the Telegraph, Mourinho also disliked Casillas for a number of other reasons: telephoning Xavi after a Clasico, disagreeing with the manor’s tactics and even for having a journalist as a girlfriend.
In his biography, The Special One, Diego Torres wrote: “Mourinho saw him as a grass, a mole, a traitor and an egoist.”
After Mourinho was caught on camera making unguarded comments suggesting Eto’o was older than his official age, the Cameroon striker reacted badly.
"I have Eto'o but he is 32 years old, maybe 35, who knows?” Mourinho said, off the record, back in 2014.
Eto’o reacted by calling Mourinho a “fool” and a “puppet”.
"Thanks for informing me that it will be my last World Cup,” Eto’o was quoted as saying by the Guardian at the time. “Today, I am 33 years old. And it is not because a fool called me an old man that you must believe it.”
However, the two men eventually patched things up and, after a goal for Chelsea against Tottenham, Eto’o celebrated with a brilliant ‘old man’ celebration.
Luiz fell out of favour at Chelsea after Mourinho returned to Stamford Bridge in the summer of 2014, with the Portuguese coach preferring to deploy John Terry and Gary Cahill in central defence instead.
After selling the Brazilian to PSG, Mourinho added insult to injury by admitting the versatile defender wouldn’t be missed at Stamford Bridge.
“Last season he was not regular first-choice for us,” Mourinho was quoted as saying by the Telegraph. “David did important things here, was always a good professional and we will miss him as a good guy when he was with us. But from a football point of view we believe our squad is stronger this season than it was.”
Needless to say, this comment upset Luiz, who exacted his revenge by taunting Mourinho on the touchline after PSG’s victory over Chelsea in a Champions League clash.
Mourinho fell out with many members of the Real Madrid dressing room during his final season at the Bernabeu, including his compatriot Pepe.
The defender criticised Mourinho’s handling of the Casillas situation - and his manager couldn’t resist the opportunity to bite back via the media.
"It is easy to analyse the Pepe thing,” Mourinho was quoted as saying by BBC Sport in 2013. “His problem has a name, and it is Raphael Varane.”
Mourinho, there, suggesting Pepe was only angry because he’d lost his first-team place to a 20-year-old.
The relationship between Mourinho and Ronaldo became so strained in 2013 that the two men almost came to blows after a dressing room disagreement.
Following a Copa del Rey quarter-final match against Valencia, Mourinho blasted his compatriot for his lack of effort tracking back.
"The player, unable to contain his fury, got up from the bench where he was changing and began shouting," Guillem Balagué revealed, per the Mirror, in his autobiography on the three-time Ballon d’Or winner. “After everything I’ve done for you, this is how you treat me! How dare you say that to me?
"Many of the players who witnessed the scene had demanded that Mourinho, either directly or through the coaching staff, ask Ronaldo to defend more.
"The situation escalated to the point where Cristiano had to be held back (some sources say by Casillas, others say by Arbeloa and Khedira, or even Sergio Ramos) to prevent the pair from coming to blows."
Five stars who fell out with Pep Guardiola
Guardiola felt that, in order to get the best out of Lionel Messi, he would need to either move or drop Ibrahimovic altogether.
Messi was moved to a central position, from the right flank, and the relationship between Guardiola and Zlatan deteriorated rapidly.
In his autobiography, Zlatan went to town on Guardiola.
In one of the chapters, the current United striker revealed: ”Guardiola was staring at me and I lost it. I thought, 'there is my enemy, scratching his bald head!' I yelled: 'You haven't got any b***s!' and worse than that I added: 'You can go to hell!' I completely lost it, and you might have expected Guardiola to say a few words in response, but he's a spineless coward."
Saturday should be interesting…
On his promotion to the Barcelona first team, Guardiola axed both Deco and Ronaldinho from his squad.
He felt they were both in decline and, crucially, were a negative influence on Messi.
Deco was sold to Chelsea and, judging by comments he made to Marca in 2012, doesn’t hold a particularly high opinion of the current Manchester City head coach.
"Mourinho is the best coach in the world because he's won everywhere and that's very difficult. He has to win the Ballon d'Or,” the former Portugal international commented. “Guardiola was a good coach at Barcelona but I still say that he didn't invent anything. To properly judge him you would have to see him elsewhere, at other clubs”.”
Yaya Toure’s heart must have sank when it was confirmed that Guardiola would become City’s manager from the start of the 2016-17 campaign.
It was Guardiola who sold Toure while managing Barcelona in 2010, and there’s no realistic chance of the 33-year-old featuring for City this season due to his questionable work ethic.
Toure’s agent has also taken several pops at Guardiola over the years.
"Pep is a great coach. But he has won the title at Barcelona and Bayern Munich,” Dimitri Seluk was quoted as saying by Sky Sports in January.
"The truth is that my grandfather would win the title with Barcelona and Bayern Munich because they are big clubs with great players.
"I'd like to see Pep take a club that is eighth or ninth and make them champions."
Guardiola irritated Schweinsteiger after selling him to Manchester United in the summer of 2015 after questioning his fitness levels.
The Catalan told reporters the World Cup winner could be a success in the Premier League, but only if he stayed fit.
Schweinsteiger, per the Daily Mail, hit back by saying: “I’m here now to make an impact, a good one, and I'm looking forward to playing more games and getting better with my fitness levels.”
Guardiola’s decision to sell the German midfielder now appears entirely understandable and it’s now Mourinho who’s in the 32-year-old’s bad books.
After Guardiola left Bayern Munich this summer, Franck Ribery has spoken of his immense relief on several occasions.
“I feel free and I am fully motivated. Carlo Ancelotti is a gift for the club. Since Ancelotti has arrived, I feel more trust,” the French winger was quoted as saying by the Daily Mail.
“He's a great coach. I need people like him, Jupp Heynckes or Ottmar Hitzfeld. I need this trust in me, to play off my strengths.
“It all depends on things like trust, respect and proximity. Then I cannot only give 100 percent - I can give 150 percent.”
In a bid to diffuse the situation, Guardiola told reporters: "About Ribery I must say I love him very much.
"It's good that Ribery says I'm young. I'm here to learn."
Who will win the Manchester derby this weekend? Post your score predictions in the comments below.