Jose Mourinho is not prone to regrets, but even he might wonder what might have been when he takes his seat in the Chelsea dugout on Saturday.
On the other side of the East Stand tunnel will sit Manchester United boss Louis Van Gaal, Mourinho's former mentor now in the job that he once appeared desperate to hold himself. Not that he would admit it, of course, and there are several sides to the story.
If you believe Spanish journalist Diego Torres’ book on Mourinho’s time at Real Madrid, the Portuguese sobbed when he found out David Moyes had been appointed as Sir Alex Ferguson’s successor in 2013.
Mourinho’s own account is that he had known for months about Ferguson’s retirement but only ever had eyes for a return TO Chelsea and would have turned down an approach from Old Trafford.
And there is the version at Old Trafford, where they insist Mourinho was never in the running to replace Ferguson anyway, mainly due to a feeling that his brashness and public courting of controversy did not fit with the ‘United way’.
For many United fans, the timing seemed perfect two years ago when Ferguson stepped aside just as Mourinho was about to become available. It was an open secret that he would be leaving Real Madrid and, at least officially, he had not signed anything with Chelsea.
Mourinho, it appeared, had been courting the job for years with his deferential praise of Ferguson and he had appeared to treat Real Madrid’s Champions League tie with United that March as an audition.
Ahead of the second leg at Old Trafford, he spoke in glowing terms about United and even gave an exclusive interview with MUTV. It was as though he was trying to curry favour with the United fans and boardroom.
In the aftermath of Real Madrid’s progression - in part thanks to a controversial red card shown to Nani - Mourinho declared: “The best team lost. We didn’t deserve to win but football is like this.”
It was not a typical Mourinho observation, and for many it was another sign that he was positioning himself to become Ferguson’s successor.
At the time, the pair had an excellent relationship. They were close enough to have shared tea in the Lowry Hotel when Real Madrid faced Manchester City earlier that season - but that bond was not extended when Ferguson anointed his successor.
Moyes had never won a trophy when he was appointed as United’s manager. Mourinho, who is just a few months older than the Scot, had already won two Champions Leagues, seven league titles in four countries and 10 other cup competitions.
It is in that context that Torres claims Mourinho felt betrayed by Ferguson and "crushed" when he realised he would not get the job after two days of frantic phonecalls to his agents from the Sheraton Mirasierra hotel.
Mourinho has denied this account, but what’s certainly true is that no-one at United ever picked up the phone to make any enquiries about his availability.
He was considered, but there was a nervousness about Mourinho’s propensity to pick a fight - he had poked then Barcelona assistant Tito Vilanova in the eye in 2011.
“A United manager would not do that. He’s a really good coach but that’s as far as I would go really,” was Sir Bobby Charlton’s assessment of the incident - one reflected in the rest of the United boardroom.
While Mourinho now talks of building a dynasty at Chelsea and staying for the next 10 years, there was also a feeling that he was not a top-to-bottom manager - like Ferguson, Moyes and indeed Van Gaal - who would pay attention to the academy.
Had Mourinho been available a year later, United might have considered him the perfect man to help the club recover from the Moyes disaster. He could have created more iconic Old Trafford memories to add to his famous charge down the touchline when his Porto side won there on their way to Champions League glory in 2004.
The chance, if you can call it that, appears to have passed. "There's no other country, club or investment that motivates me. I will only leave when Mr Abramovich calls me to tell me it is over," said Mourinho in February.
Perhaps he is leaving himself some wriggle room. If one day Abramovich casts Mourinho aside - and he’s done it once before - he could belatedly take over at Old Trafford.
Mourinho has not completely stopped his charm offensive. On Friday he said: "To play against Manchester United is to play against one of the biggest clubs in the world and one of the best in the Premier League. This is what everyone wants. It is a special fixture."