On Saturday, Brendan Rodgers was undone by his fellow countryman.
Having gone unbeaten in the first 18 league games last year, it was a sobering reality check for the Hoops.
Rodgers picked his team with Greece on his mind, making five changes; James Forrest, Kristoffer Ajer, Tom Rogic, Oliver Ntcham and Odsonne Édouard were all rested following the return leg against AEK Athens in the Champions League qualifying round.
Champions League journey a non-starter
Defeat to Hearts was a blow, but the Celtic manager knew failure to progress in Europe would be a catastrophe.
In the event, the Scottish champions failed to get the job done against AEK Athens, losing 2-1 on the night and 3-2 on aggregate to send them crashing out of the Champions League and into the Europa League qualifiers.
After the excitement of colliding with continental heavyweights Bayern Munich and Paris Saint-Germain last season – and Barcelona and Manchester City in 2016 – Celtic now face either Latvian side Spartaks Jūrmala or Lithuania's Sūduva to prolong their involvement in European competition this season.
For Rodgers, who has led Celtic to unprecedented domestic heights – an historic ‘Double Treble' – during his time at the Parkhead helm, the failed assignment in the Greek capital represents the crushing nadir to what has been the most turbulent and trying time of his reign.
As is almost inevitable in modern football, an underwhelming summer in the transfer market has led to tension between Rodgers and the board.
The club secured the permanent signing of Édouard from PSG following a productive loan in Glasgow last season but, outside of veteran left-back Emilio Izaguirre returning after his year-long sojourn in Saudi Arabia, there has been little to shout about by way of incomings.
During the defeat to AEK, Chris Sutton led the criticism with a typically withering assessment of the club's transfer activity. “They’ve spent £2million on disco lights at Celtic Park for Champions League nights,” the former striker said during his commentary on the game.
“They might not get there. Invest in the playing squad.”
They didn't get there and they will most certainly need to invest.
That is easier said than done at this late stage. Stuart Armstrong‘s departure to Southampton coupled with missing out on chief transfer target John McGinn to Aston Villa has laid bare the need for fresh talent but, following a peculiar period of inertia, the club now face the prospect of scrambling to get last-minute deals over the line.
Boyata issue needs to be resolved
Then there is Dedryck Boyata who, after three years at the club, is in danger of completely alienating the fanbase – if he hasn't already. The Belgian international defender has yet to play this season having been subject of a £9million bid from Fulham in the closing stages of the Premier League transfer window.
According to Rodgers, Boyata was fit to play in Greece despite the player posting a video on social media pre-match of him receiving treatment, painting a bleak picture of a disgruntled player refusing to play while agitating for a move away.
To a passionate set of supporters, who value the badge above all, Boyata's actions are impermissible.
With Ajer suspended, Rodgers was forced to field a back three of Jozo Šimunović, Jack Hendry and Mikael Lustig, whose combined troubles only brought Boyata's absence into sharper focus. Hendry, a January signing from Dundee, was beaten in the air for AEK's second goal while Lustig endured a torrid night.
The Swede was turned inside and out by Niklas Hult in the build-up to Rodrigo Galo's opener before giving away the free-kick that led to Marko Livaja's decisive second.
Sutton branded Boyata a ‘disgrace' and, while the full story is unclear, resolving the issue has to be item one on Rodgers' agenda. The last couple of weeks have inflicted arguably irrecoverable damage on Boyata's relationship with the club's fans, driving the player to a point from which there may be no return.
Whether he stays will be Rodgers' decision. Does the former Liverpool boss banish Boyata to the reserves for the seemingly unprofessional manner in which he has tried to clinch a move to England, or does he gradually reintegrate a centre-back whose experience will prove valuable throughout another demanding season?
Indeed, following Tuesday night's ignominous European exit, Celtic fans took to social media to castigate the former Manchester City defender who, as some pointed out, has found a new lease of life in Scotland having failed to break into the first team at the Etihad Stadium.
It remains to be seen if Boyata can come in from the cold but, judging by the current temperature on Twitter, a return would be met with vilification.
Spending made more difficult by Greek tragedy
The other issue is investment. Shattered Champions League dreams will cost the club roughly £40million and now, set to the backdrop of growing disillusionment among supporters, the board will be more reticent than ever to loosen the purse strings.
A subdued end to the transfer window – which, unlike south of the border, remains open until August 31 – may aggravate the simmering tension between higher-ups and Rodgers who, following the defeat in Greece, made clear his thoughts on the club's disappointing transfer business: “It's pretty obvious, you have to keep progressing and keep getting stronger.”
With regards to on-pitch matters, Celtic have had better days but they still have the finest squad in Scottish football.
Rodgers now faces his most unenviable challenge since being appointed in 2016; re-motivating a dejected squad ahead of a hectic period in which the club will play five times in two weeks, starting with a trip to Partick Thistle in the League Cup on Saturday and culminating with Rodgers' first Old Firm derby against a Steven Gerrard-led Rangers on Sunday, September 2.
The last two derby meetings have produced an aggregate score of 9-0 in Celtic's favour. It's difficult to envisage another merciless annihilation of their bitter rivals in a few weeks' time.
Following a turbulent few weeks, Rodgers would be happy just to avoid further embarrassment.