IT began with red faces beneath the Rock of Gibraltar. And ended with some of the most exalted names in European football, those with an embarrassment of riches to call upon, being made to feel decidedly hot under the collar.

Celtic's 2016-17 continental campaign might have lasted less than five months but it has been quite an excursion.

While the supporters have enjoyed their city breaks - in a few isolated cases, a little too much - for the players themselves it has been a voyage of self-discovery.

Read more: Forgotten man Gary Mackay-Steven hoping to follow in Armstrong's footsteps as manager's Midas touch pays off again

Only five men who started that day against the part-time firemen, policemen and taxi drivers of Lincoln Red Imps - Scott Brown, Craig Gordon, Moussa Dembele, Erik Sviatchenko and Tom Rogic - and were culpable for a result which was called the worst in the club's history, and one of the biggest shocks in the continent's history, were still lining up as the strains of Zadok the Priest rang out around the Etihad Arena on Tuesday night.

But four of the other starters against City - James Forrest, Stuart Armstrong, Patrick Roberts and Mikael Lustig - were on the bench, and the other two - Jozo Simunovic and Emilio Izaguirre - were at the club too. Put the two performances against each other, and they are hardly recognisable.

What Brendan Rodgers has managed to eke out of this same group of players in the course of the two qualifying ties, against Astana then Hapoel Be'er Sheva, then six Champions League group stage matches, has been something to behold. This stands, despite the fact that the Parkhead side's European adventure ends before Christmas with them rock bottom of their section with three points to their name.

Aside from a 7-0 clouting at the Camp Nou in the opening match day, Celtic have shown individual and collective growth as they have gone along.

Read more: Forgotten man Gary Mackay-Steven hoping to follow in Armstrong's footsteps as manager's Midas touch pays off again

City fielded a weakened team on Tuesday night in the full knowledge they had already qualified, but Celtic can at least tell themselves that had they sneaked the wins which they probably merited in both Moenchengladbach and Manchester then they would be marching on to the last 16 with seven points to their name this morning. It cannot be said that the Scottish champions were uncompetitive in this group.

While he never thought that the entire campaign was going to be stopped before it ever got started, Rodgers said yesterday that, with the benefit of hindsight, in some ways that Lincoln Red Imps experience was actually a blessing.

"From the first game we played on the Astro in Gibraltar it’s fair to say we came a long way," said the Celtic manager. "It was a nice little wake up for the players. It told me a couple of wee things early on about the fragility of the team. It sharpened the teeth in many aspects of what we wanted to do."

Fast forward five months and the difference is frightening. Not only are Celtic's first team becoming finely tuned when it comes to their manager's tactical outlook even at venues like the Etihad, but Rodgers took in the 3-2 defeat in the Uefa Youth League against the same opponents and noticed that those same first principles, including ferocious pressing and calm passing from the goalkeeper out, are now being instilled among the club's youth players.

"Listen, there are different ways to play football - but my teams have always been aggressive when they defend, press the game, never give the opposition a minute’s peace and when they have the ball they show the courage and bravery to play the game to a high level," said Rodgers.

"Slowly we’re starting to see that. But it’s okay doing it in other games domestically but can you go and impose that in the biggest jungles in the world? Going to a team that no matter the changes they made, would still finish towards the top end of the Premier League, we imposed our style, calmness and quality on the game. It shows we have grown and developed and that is what this year was probably going to be about.

“Our biggest disappointment ends up being Moenchengladbach at home," he added "Actually two away draws against Man City and Moenchengladbach is very, very good. And we had possibilities to win. That gives you great confidence going forward. A year further down the line hopefully we can make the next step."

Read more: Forgotten man Gary Mackay-Steven hoping to follow in Armstrong's footsteps as manager's Midas touch pays off again

What exactly that next step turns out to be promises to be fascinating. More time on the training ground will help, although it may prove more difficult than the Northern Irishman thinks to maintain this forward momentum now the adrenaline shot of Champions League football has gone for the season. Another conundrum will be adding the few pieces of quality the squad needs, without inhibiting the current crop.

Stuart Armstrong is perhaps the perfect case study A player who appeared riven with self doubt stuck out on the left side under Ronny Deila is now blossoming into the figure people thought he could become and more. He, Scott Brown and Tom Rogic now operate like a little triangle in midfield, but how might bringing in someone like rumoured transfer target James McCarthy upset that equilibrium?

“Players have to feel they can do something," is Rodgers' mantra. "It’s no good that Moussa Dembele knows that I know he can jump as high as this room. What makes him jump? There are plenty of people in life who can do things but don’t do them. Stuart Armstrong can run for 24 hours a day. But sometimes you have to think and run."

One thing is for sure, that Celtic under Rodgers will never run away when it comes to locking horns with Europe's big guns. "You can sit off and suffer and sit in the box for 85 minutes, but I like to be the aggressor," he said. "If you stand off good players they will run rings round you all day. But how good can they be if you go right up against them? Every now and then they will earn their £200,000 a week and play around you a couple of times – that’s okay. Let’s see if they can do it again and earn their money.” However much Celtic are paying their manager, he is certainly earning his.