Jose Mourinho has mentored several future managers; Brendan Rodgers, Aitor Karanka and Andre Villas-Boas among them. But he has always insisted his long-time assistant Rui Faria rated above them all.
'If one day I have to speak about disciples, the real one is the one that has been with me since 2000,' he said shortly after taking over at Manchester United.
'The one I really feel thinks like me, is adapted to my way of coaching, the one with most similarities to me, even in terms of personality.'
For the first time since the turn of the century, Mourinho has had to work without his closest comrade and confidante this season, Faria leaving United and his long-time boss in the summer after almost 20 years of unprecedented success at Porto, Chelsea, Inter Milan and Real Madrid.
The reasons are still mysterious – though Faria's camp insist it was nothing other than a need to have a break and one day become a manager in his own right – but the impact is clear. Mourinho looks lost, isolated and nervy without his mate.
Watching Faria over the years was like often like witnessing a perfect mini-me Mourinho.
If the boss hadn't the energy to berate the officials, Faria would step in with interest. The verbal attack during the back end of Mourinho's first home Premier League defeat at Stamford Bridge was extraordinary.
If Mourinho was unable to deliver his sulky, sarcastic one-liners at press conferences, Faria filled in if his boss was banned or had been sent to the stands during the game.
When United's Ander Herrera was sent off by Mark Clattenburg during a disappointing goalless draw against Burnley in 2016, it was Faria who addressed the media and pouted: 'I just want to say fantastic work from the referee.' It could have been Mourinho sitting there.
And yet there relationship was far more than one-dimensional master and servant. Faria was intelligent enough to act as a conduit between the manager and players if Mourinho was in one of his harsher moods.
Mourinho spent most of last season lambasting Luke Shaw, Faria would offer the left back comfort and advice as a counter-measure. Who is doing that job at United this season if Paul Pogba or Anthony Martial have their feathers ruffled?
Of course, Michael Carrick is on board but despite the respect Mourinho has for the United legend, they can't build a 'band of brothers' relationship overnight.
Faria was with Mourinho from the beginning. In 18 months, they were unknowns on the Portuguese League, together helping Porto beat Celtic in the 2003 Uefa Cup Final. A year later, they were kings of Europe, Porto winning the Champions League before the architects moved to England and ended Chelsea's 50-year wait for a league championship.
Faria visibly fought Mourinho's corner in the dugout, an irritating figure to opponents. The manager always knew as he fought the world that his No 2 would always be in his corner and that gave him incredible confidence to keep fighting.
In Manchester, a lonely life in an apartment at the Lowry Hotel was made more bearable for Mourinho by having his best friend alongside him, even if his family were 200 miles away in London.
'It's important to have people around that identify themselves with me,' he explained in 2017. 'If I need to be in a meeting for a reason and unable to be in total control, Rui has the knowledge of how I think.
'He lives two minutes away from me, we have dinner together pretty much every day with my other staff (goalkeeping coach Silvino Louro). Because we are without our families, it makes up partners every day.'
My assistants all live in the same building, and their building is two minutes away from the hotel where I'm staying.'
Mourinho knew one day that Faria's ambition and the workload put on him would one day see a parting of the ways. Faria is still only 43 and had hankered after being a manager for some time. Friends say the big surprise is that he was able to support Mourinho for so long, not that he's now gone.
The Special One thinks Faria has the ability to do better than Villas-Boas or Rodgers, both of whom have had success as managers. 'If one day I have to choose my successor if you want to call it that, it is Rui Faria. The one with more potential than anyone.'
Yet the fact that Faria hasn't yet taken up another job has led to speculation the pair had a fall-out – the one man Mourinho couldn't afford to fall out with.
There is no firm evidence for that, insiders say Faria still talks to his old boss and spends a lot of time in London where his daughters are understood to be studying. It was simply a case that after 18 years on the road with Mourinho, Faria wanted to prioritise family first, and eventually start his own managerial career.
Like Brian Clough without Peter Taylor, Mourinho is struggling without him. Respected people in the game are questioning whether Mourinho has lost the mental strength – once his forte – to succeed during the very highest pressure points.
At the end of last season, Mourinho explained a coaching revamp to cope with Faria's departure. 'I will organise my coaching staff in a way where the assistant-manager figure doesn’t exist,' he said.
He was essentially saying Faria was irreplaceable and the first month of the season at United tells you why. Maybe he'll ask him back. Maybe he'll find a way to be successful on his own. But there's no doubt the loss of Faria has had a huge impact so far.