Pep Guardiola and Erik ten Hag have always been effusive in their praise of one another.
The Dutchman described the two years he worked alongside Guardiola at Bayern Munich as 'like winning the lottery'.
And when it was rumoured Ten Hag was going to take the Manchester United job back in April, Guardiola joked his old colleague should come to City instead.
What is clear is that the Ten Hag vision of playing the game that is slowly unfurling at Old Trafford, and the meticulous attention to detail required to realise it, are influenced by those formative years watching Guardiola hard at work on the next training pitch.
It will be strange for both men to find themselves in direct opposition - for the first time - in Sunday's Manchester derby at the Etihad.
Given Ten Hag is 52 and Guardiola is 51, it's hardly master taking on apprentice but it could well feel that way in the latest examination of the Dutchman's Old Trafford revolution.
It will take something special for United - for all their renewed promise in recent weeks - to get the better of an all-conquering City side over six years in the maturing under Guardiola.
City remain unbeaten after seven Premier League games, sitting only a point behind leaders Arsenal, and Erling Haaland, with 14 goals already in all competitions, has already confounded those who believed he'd struggle in English football.
But United's counter-attacking punch has floored City in the past and there is considerably more optimism around Old Trafford than there was six weeks ago when they were humiliated by Brentford.
There are the green shoots of recovery as Ten Hag's methods take ever greater effect and this derby promises to be a whole lot closer than last season, when City were utterly dominant in a 2-0 win at Old Trafford before smashing Ralf Rangnick's no-hopers 4-1 in the return.
Ten Hag raised eyebrows when he left Go Ahead Eagles shortly after securing promotion into the Eredivisie in 2013 to become coach of Bayern's second team, who played in regional football in Germany.
But there was one principal reason why Ten Hag, then 42, was willing to swap senior football for youngsters and that was the opportunity to observe Guardiola at close quarters.
At Bayern's old training base, the first team and second team pitches were adjacent to one another, offering the two coaches ample chance to compare notes on tactics.
Then there were the private conversations between these two kindred spirits behind the scenes about their football philosophies and aspirations.
Ten Hag was a studious and diligent man anyway, as former players at Go Ahead Eagles attest to, but working alongside Guardiola gave further vindication to his methods.
'Erik is very disciplined and people have to buy into that and have that work ethic,' Steve McClaren, part of Ten Hag's backroom staff at United, told the Daily Telegraph.
'Tactically he's outstanding. He worked with Pep at Bayern Munich and took in his philosophy too. They called him 'Mini Pep' out there.'
While Guardiola was winning Bundesliga titles but not quite replicating the Champions League successes of his time with Barcelona, Ten Hag led the second team to top spot in the 2013-14 Regionalliga Bayern.
However, they weren't promoted after losing a play-off to Regionalliga West champions Fortuna Cologne thanks to a 94th-minute goal in the second leg.
'I learnt a lot from Guardiola,' Ten Hag reflected in 2019 when Ajax manager. 'His philosophy is sensational, what he did in Barcelona, Bayern and now with Manchester City, that attacking and attractive style sees him win a lot.'
In an interview with Voetbal International in 2020, he said: 'Nobody will ever forget Guardiola. He has won so many trophies for a start - but it is mainly about how his teams play.
'Guardiola only wants to win games with beautiful football, just like his teacher Johan Cruyff.'
So when Ten Hag left Bayern for Utrecht in 2015, his video recordings of Guardiola's training sessions all saved away for reference, his tactics and coaching techniques were very similar.
Combined with a ruthless streak that demanded discipline from his players but ultimately commanded respect, it led to considerable success at Ajax, where they won three league titles and reached the Champions League semi-finals.
Ten Hag's Ajax would have played Guardiola's City in that semi-final but Tottenham prevailed in their all-English quarter-final by the slenderest of margins.
Clearly Ten Hag had an effect on Guardiola, too. As he said around the time of Ten Hag's appointment at United: 'We spoke quite regularly and he is just an incredible person and human being.
'I was surprised by how humble he was. For his qualities, just take a look at his Ajax team in the last years. It's a joy to watch.'
In another press conference on April 1, when speculation United would choose Ten Hag was at fever pitch, he even said: 'If I was 100 per cent, I would call Manchester United and tell them: 'Guys, you have to take him.'
They did and now Guardiola may have to deal with a resurgent rival under a manager he clearly has tremendous respect for.
Ten Hag has said previously that Guardiola made a mistake during his first season with City because he 'underestimated the power and speed of the Premier League'.
'He realised you can't play the football he loves without a couple of physically strong athletes,' he added.
After saying that, you'd hope Ten Hag doesn't fall into the same trap during his first season with United. Up next, the power and speed of his old friend's City team.