Usually, Jose Mourinho would be looking forward to a return to his old club: he enjoyed great success with the Portuguese side and still has many friends at the Estadio do Dragao.
This occasion is slightly different, however; Iker Casillas will start in goal for Porto and it's fair to say that he and Mourinho have their differences.
When Mourinho left Inter for Real Madrid, his reputation in football was sky high. He'd created title winning sides with clubs that had previously endured a barren spell with trophies.
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He was tactically astute, charismatic but most importantly, seemed to have a perfect relationship with his players.
At Real Madrid, his ability to man manage was heavily questioned.
Players too big for the club
Rather ironically, considering his is the largest in modern football, Mourinho hates big egos. At all clubs he's managed, he's banished any suggestion of stardom and insisted that all in the squad were equal. The difference at Real Madrid was that there were superstars - you either had to embrace it or you'd leave.
There was not a more iconic symbol of modern day Real Madrid than Iker Casillas. He'd overseen several league titles and Champions League titles, he was almost untouchable at Real Madrid. Mourinho didn't like that. It's thought Casillas believed Mourinho dropped him to send a message to the rest of the squad - as if to say 'If Casillas can be dropped then so can anybody.'
Breakdown of relationship
Mourinho reiterated that repacing Casillas with Diego Lopes was purely tactical.
"I like Diego Lopez as a goalkeeper more than Casillas,” explained Mourinho, according to Goal. “It's not personal. I like a goalkeeper that comes out and dominates the airspace and plays with his feet. Iker is fantastic under the posts.”
Jose's words may seem plausible, but it's hard to believe that the decision to drop arguably the best goalkeeper in the world at the time, was a tactical one. To put it into context: Casillas has played under 12 managers at Real Madrid, Mourinho is the only one that deemed him not good enough.
It's much more likely that Mourinho and Casillas' personal relationship became untenable. Casillas recalled to Canal+ - per the London Evening Standard - how Mourinho accused him of leaking team news via his journalist wife, Sara Carbonero.
“Soon, I got injured and after two weeks, I noticed the attacks. I was called a traitor and a mole." Casillas said. “It was hard to be called a mole and I like to think that neither Mourinho nor the president Florentino Perez really believed this.”
I guess only Mourinho will ever know if he truly believed his own accusations.
Fortunes since leaving the Bernabeu
After leaving Real Madrid for Porto and Chelsea respectively, Casillas and Mourinho have savoured relative success. Mourinho won the league in his second season with Chelsea, whilst Casillas has shown glimpses of the form that saw him named in the FIFA World XI for five years running.
I doubt their reunion this evening will be a harmonious one, especially after Mourinho revealed the one regret of his Real Madrid tenure was not dropping Casillas sooner.
"Would I do anything differently if I had the chance to do the past three years over again? I should have brought in Diego Lopez after my first year. We didn't do enough to sign him. It's a real shame," Mourinho was quoted as saying by Goal at a press conference.
After hearing those comments, I doubt Casillas will need much motivation to prove his old boss wrong.