Moments after the final whistle blew, VfB Stuttgart’s players reveled in their victory alongside ecstatic fans at Celtic Park. Meanwhile, in the stadium’s premium seating, a familiar figure made his rounds, shaking hands and reconnecting with acquaintances.

More than two decades ago, Andreas Hinkel was part of the Stuttgart squad that fell to Martin O’Neill’s formidable Celtic team, a group en route to the UEFA Cup final in Seville. Five years later, Hinkel joined the ranks of the Glasgow giants, learning firsthand the enduring European reputation Celtic once commanded.

Reflecting on the past, Hinkel posed a poignant question to old friends: “What’s happened to them?” This query strikes at the heart of Celtic’s current status on the European stage.

Although the legendary 2003 encounter is often recalled in such discussions, one need not delve so far back to observe Celtic’s evident decline in European competitions. Just last season, under the stewardship of Brendan Rodgers, Celtic impressively outclassed RB Leipzig in the Champions League, advancing to the play-off round. Though they were ultimately overpowered by Bayern Munich, Celtic’s performance across the two legs suggested a promising new benchmark had been set.

While the epic 2003 clash was naturally a reference point around this tie, it wasn’t necessary to go back a generation to gauge Celtic’s marked decline on this stage.

Celtic’s Sebastian Tounekti looks dejected as Stuttgart celebrate their 4-1 victory

Celtic fans turned on Kasper Schmeichel after he was beaten too easily by two of the goals

Supporters also had their say at the start of the match when players had to clear away tennis balls thrown on to the pitch in protest

Last season, under Brendan Rodgers, the team dismantled RB Leipzig in the Champions League as they progressed to the play-off round. While Bayern Munich would prove a game too far, Celtic performed valiantly across both legs. It felt like a new standard had been set.

Stuttgart are an accomplished side. They are not, though, as good as Leipzig or Bayern from last year.

The reason Celtic fell to a comprehensive 4-1 defeat in this week’s first leg had more to do with their regression and less to do with the standard of their opponent.

Since coming within seconds of taking the game in the Allianz Arena to extra-time, the Parkhead club have gone into free-fall.

O’Neill has done a heck of a job of twice getting the side back into the Premiership title race and securing progression in Europe, but no-one is being fooled.

A struggling side have required stoppage-time goals in three successive domestic matches to get the job done.

You have to praise the spirit O’Neill has instilled since returning. You must also acknowledge that this current Celtic side are not a great team habitually pulling off heroic acts. They are an ordinary one who are riding their luck, with a brilliant manager at the helm.

Thursday’s mauling to a superior team underscored the point. It said much that the only cause for celebration among the home fans came when Benjamin Nygren capitalised on an error to score.

O’Neill was angry with fans who threw balls on the pitch and said they ‘needed their heads examined’

Skipper Callum McGregor’s despair is clear to see as Celtic reflect on a 4-1 defeat

Stuttgart’s Bial El Khannouss takes a selfie after scoring a double at Celtic Park

Celtic created next to nothing on the night. They were second best all over the park. They were fortunate Stuttgart didn’t humiliate them by scoring six or seven.

Only someone who has been living in a cave for the past year could possibly have been surprised by this.

From a position of strength and promise that night in Munich, the club’s hierarchy have overseen a staggering nosedive.

They decided against fixing the roof while the sun was shining. And the water is now cascading through it.

A failure on the board’s part to adequately support the football department in successive transfer windows led directly to elimination by Kairat Almaty in the Champions League qualifiers.

The financial implications of this ineptitude were manifest in the recent interim financial results which showed revenue dropping by 28.9 per cent.

Celtic’s shortcomings have not, however, been restricted to a failure to source adequate replacements for the likes of Kyogo Furuhashi and Nicolas Kuhn.

Squad management also encompasses knowing when players should be moved on – either on account of their contractual situation or their age. The club’s hierarchy have seemed blinded to the issues that everyone else sees in the rear-view mirror.

Schmeichel failed to convince as El Khannouss scored the first goal for Stuttgart

The veteran keeper was also badly at fault for Jamie Leweling’s goal which killed the game at 3-1

Schmeichel looks shellshocked on a night when some fans decided to boo the veteran keeper

This week, O’Neill insisted that Reo Hatate’s poor recent performances should be attributed to a loss in form rather than a bad attitude. Be that as it may, but the midfielder hasn’t been the same force since Celtic rejected Udinese’s advances last summer.

It’s been the same story with Daizen Maeda since he was denied the chance to move to Wolfsburg.

The pair have been at Celtic for four years now. They would have expected to have moved on to clubs in bigger leagues by now. Is it any surprise that their levels have dropped?

The case of Kasper Schmeichel has similarities. Ahead of turning 40, the Dane’s next move will be retirement this summer.

He was a brilliant goalkeeper in his day and filled the void left by Joe Hart extremely well for a season.

This campaign has evidently been one too many, though. He was culpable for two goals on Thursday, adding to a multitude of errors in other big matches.

Celtic, characteristically, hoped they would get away with having Schmeichel in goal when all the evidence pointed to the need to get ahead of the problem. Again, they’ve been caught short.

While you could understand O’Neill’s anger at a small number of fans booing the keeper in midweek, you could easily appreciate why such an unedifying spectacle played out.

Schmeichel has become emblematic of the penny-pinching, make-do-and-mend approach which permeates the entire club.

Martin O’Neill looks like a beaten man for once after his team crashed to a 4-1 defeat

You would have needed a heart of stone not to have felt something for him as Stuttgart ran riot. It’s hardly his fault that no one above him made the decision which would have rightly seen him taken out of the firing line.

The booing of the veteran keeper was hardly incongruous with everything that had gone before.

With the Green Brigade still banned, the atmosphere from the outset was flat. The early disruption of the game when tennis balls were thrown onto the pitch as part of a protest certainly didn’t help the home side make a fast start in the contest.

The fabled 12th man who was such an advantage for O’Neill’s great side back in the day has temporarily left the building.

Unless interim chairman Brian Wilson can broker a peace deal with the various disaffected groups, the remainder of this season will be played out amid an air of ongoing acrimony.

After a string of last-gasp heroics, the sobering defeat to Stuttgart was a reminder of the bigger picture. Celtic, a club divided, is in a dark place. The hierarchy is solely responsible for taking them there.