Their relationship, at least from the outside, did not always appear to be as straightforward as that of manager and player.
Martin O'Neill was often regarded as something of a mentor for Neil Lennon in the Northern Irishman's playing days, although the former Celtic manager says he has never offered advice to Lennon – and he doesn't plan on starting now.
Lennon has taken Celtic into the last 16 of the Champions League where a glamour tie against Juventus awaits this evening.
O'Neill himself never managed to achieve what he regarded as the holy grail of getting the club out of the Group Stages, but there are no green eyes when he looks to Lennon this evening as his fellow-countryman takes his place in the Hoops dugout.
"I do not have one ounce of envy for anyone at Celtic," said O'Neill." I am delighted, I am thrilled they are doing so well. It is a marvellous achievement. I am delighted for Neil that it is going so well. I really am.
"But I would have no advice for Neil whatsoever, not that he'd take it anyway. He never took it from me when I was his manager, so I doubt he'd take it now. Seriously, I'd have no words of wisdom. He's doing very well on his own without me or anyone else meddling."
The duo last spoke a few weeks back at the LMA Forum, a conference attended by the majority of the Premiership managers as well as the likes of Giovanni Trapattoni and Fabio Capello.
O'Neill's stay was brief due to club commitments, but he did get the chance to shoot the breeze with Lennon, with the subject of Juventus inevitably high on the agenda.
"I had the chance to talk to him, very briefly," said O'Neill. "We had a replay on the Tuesday night against Middlesbrough so, to my regret, I was not able to stay on, but I did chat briefly to Neil – and I wished him all the very best for this tie.
"Can he do it? Can Celtic go through? Well, why not? It will not be easy. Juventus have reinvented themselves and they are back at the forefront of the game. But Celtic are at this stage on merit and I'm sure Neil will have a few ideas about how he wants to go about it."
O'Neill will not be at Celtic Park for tonight's game, or at least not physically. The Irishman has been invited on more than one occasion to return as a guest of the club, but has so far refused, insisting it is only fitting someone else should hog their time in the limelight now.
"I have been invited for the game and I have been invited along to other nights," he said. "I always find it a very kind gesture, but I feel that the torch has been passed on.
"It is not my time, it is someone else's turn to do it their way. I would never want to sound sycophantic but I had five of the greatest years of my life with Celtic, and I really mean that. They were fabulous years for me at the football club and I would also include my family life in that, too.
"The Celtic job is one of the best in the world, and I really, really mean that. It is a fabulous football club. I feel honoured to have managed it, to be part of that rich history and I don't say that flippantly.
"The sense of history, the grandeur and the sheer magnitude of the club – it all comes home to you on those marvellous European nights under the floodlights at Celtic Park. There is really no place like it on a night like tonight."
His reluctance to return to his former stomping ground should not be mistaken for a lack of interest in the fortunes of his old club. O'Neill won't be in Glasgow this evening, but he will be an avid spectator as Celtic attempt to give themselves a strong result to take to Italy next month.
"Since Celtic's European campaign began, I have missed just one game because I had to go and watch Middlesbrough as we had them in the League Cup and that was my only chance to see them.
"That was the night Celtic played in Barcelona, but I have seen every other game in its entirety. And I'll be there in front of the TV tonight, willing them on. Absolutely."
Participation at this stage of the tournament puts Celtic on the map for so many reasons. Of immediate note is the fact they are mingling in elite company having got through the Group Stages, and another is the financial strength it lends them going forward.
O'Neill himself played a significant part in restoring Celtic's reputation on the continent after the club had toiled for more than a decade on that front, and he believes the real pressure for Lennon came not when he was embroiled in the challenges of the Group Stages, but in the qualifying games that made sure he got there.
"To me, the big achievement for Celtic was getting into the group in the first place," he said. "Mark my words, getting through two qualifying games to get to there is no easy task.
"You have players who might not be fully match-fit, the competitive games with so much riding on them can come very early and you are plunged right in. Getting through those and getting into the Group Stages deserves a huge amount of respect. But to come out of that group, a group that had Barcelona and Benfica – who are no mean team at all – to come out of that was extraordinary."
And O'Neill knows just how important it was that the club have finally shaken off their away hoodoo in the competition. It was a monkey on their backs for a generation, a psychological precedent set perhaps that controversial night in Turin back in 2001.
"What really did it for me this season, I think, was the result in Moscow," he said. "I speak from experience when I say that I know how difficult it is to go on the road in the Champions League and win. That night in Moscow, that was a major three points.
"One point would have been great, but when they took all three you knew they'd given themselves a fighting chance."
He won't see him, but Lennon will doubtless hear him in tonight's aftermath.