Brendan Rodgers' reaction to securing a place in the history books offered an insight into how he achieved an unprecedented triumph.
In the immediate aftermath of securing back-to-back Trebles - a feat that Celtic's Jock Stein and Rangers' Walter Smith both fell just one trophy short of - the Celtic manager was focused on one imperative: improving.
Even during the celebrations and, perhaps, even a sense of relief at finally securing the prize, Rodgers spoke about pushing his players on again.
That mantra, that commitment to self-improvement and maintaining the highest of standards, has underpinned his work from the moment he arrived at Celtic two years ago.
"The Celtic manager and the Celtic captain are on the pitch, medals round their necks, they've literally just made history, and they're talking about improving," former Celtic midfielder Kris Commons told BBC Sportsound.
"Brendan is talking about pushing these players even further next year, he's going to demand even more. That's a remarkable attribute from a manager."
How Celtic were revitalised
In Ronny Deila's last season as manager, Celtic won the league with 86 points, but lost out at the semi-final stage in both domestic cups, failed to reach the group stage of the Champions League, and finished bottom of their Europa League group.
Then Rodgers swept into Celtic and immediately raised the demands on the players, from the fitness to their application, from their preparation to their execution.
His attention to detail and the way he continually set collective and individual targets for this players resulted in the 'Invincibles'. Domestically, in his first season in charge, Celtic played 47 games, won 43, and lost none - and captured a treble.
Rather than be sated, Rodgers found a way to maintain the hunger and drive of his players. Preparation, to Rodgers, is everything.
"As a Celtic player that's the sort of thing that is spoken about in summer - the Treble," said Commons, who worked with Rodgers for one season at Celtic before leaving the club.
"Brendan Rodgers has said he believes they've got a better understanding of what he's wanting them to do on the training field. He thought they were a better squad this year, the addition of Olivier Ntcham - he's turned into a real player. Odsonne Edouard, he's another young player that has come in.
"We've seen Celtic go down to 10 men, we've seen them go a goal down, and we've seen them in tricky situations where they seem to handle it very easily. In previous years you'd be looking around thinking "OK, what's the plan now?" Brendan's always got a plan B, a plan C."
Building round Scott Brown
Last month, Celtic captain Scott Brown became only the second player - the other being Henrik Larsson - to win the PFA Scotland players' player of the year award for the second time.
He also won the Scottish Football Writers Association's player of the year award, then lifted the Scottish Cup to complete the 'Double Treble', before signing off the season with a testimonial at a packed Celtic Park.
Yet before Rodgers arrived, an injury-plagued season had led to questions about how much longer Brown could operate at the highest level.
Once Rodgers had accepted the offer to succeed Deila, he invited Brown to dinner in London, to put the first building block in place for his reign.
"He's got more professional [under Rodgers]", said Commons.
"Two years ago he was on the front page of the press with a pizza in his hand, but one of the first meetings he had with Brendan Rodgers was to say that he had to train harder.
"Rodgers would also have said to him, 'you're the Celtic captain, you've got a responsibility, this is how I want you to be'. He came out of the meeting very happy and he's never looked back since.
"In this day and age, if you want to be performing at the top level you've got to be a 24-hour athlete."
The Europe question
It is 15 years since Celtic's run to Seville and an extra-time defeat by Jose Mourinho's Porto in the Uefa Cup final.
The then-manager Martin O'Neil did not stop there. A win and a draw knocked Barcelona out of the following season's Uefa Cup. O'Neil would go on to earn another draw with Barcelona and also hold AC Milan and Bayern Munich before he left.
Gordon Strachan led Celtic to victory over Manchester United on the way to a Champions League last-16 spot. He also boasts a win against AC Milan.
Neil Lennon's side defeated Barcelona, while he also took Celtic to the last 16 of the Champions League.
However, Europe is where Rodgers - a manager who believes in controlling a game through possession and attacking opponents - has fallen short.
He has made an improvement on Deila in that he has taken Celtic to the group stages, but heavy defeats have blotted his record.
"When you put him up against Paris St-Germain, Barcelona, Manchester City, Bayern Munich, you're talking about a billion-pound squad. It's a huge, huge gulf," Commons said.
"When I was playing, the philosophy was to be hard to beat, be organised. This squad now are a far superior football team that are just lacking that bit of luck sometimes.
"He could spend £100m on the squad and still be way short.
"Certainly in Europe, their defence has been questionable. But you're up against world class strikers. We played in the Champions League with Virgil van Dijk at the back and still conceded goals."
Treble Treble and 10-in-a-row?
Commons has little doubt that Rodgers will stay and try to lead Celtic to the fabled 10-in-a-row, but squad management - balancing recruitment against player development - will be the challenge for the Celtic manager.
He has mostly relied on a core group of players, with five having played 100 times or more under Rodgers in two seasons.
However, with big clubs circling his star players, it remains to be seen how the side will evolve, which presents the challenge of how to replace them. Only Kristoffer Ajer, who was already at the club, and summer signing Ntcham have really broken into the first team as regular starters this season.
"There's a lot of people that have come in to Celtic that have not had the impact that you'd think," said Commons.
"Eboue Kouassi, Marvin Compper, Dorus de Vries, Cristian Gamboa, Charly Musonda - players they spent a lot of money on. It took Ntcham three to four months to find his feet.
"It's the nucleus of what has been there for the past four or five years that Rodgers has excelled with.
"Scott Sinclair and Ntcham are the exceptions that have done really well. The people coming in this summer will have to hit the ground running because the nucleus of the squad is playing at such a high level."
Rodgers has proven himself to be an outstanding coach and manager - not least because he has been relentless in setting new standards at the club.