Ronny Deila has some serious issues to address, urgently, both on and off the field.
His team were taken apart by Molde, a side sitting seventh in the Norwegian league, who were able to expose recurring weaknesses in the Scottish champions.
A fragility at the back has been one of the themes since the season began and though Deila can point to their defensive record being the best in the Scottish Premiership, European competition has laid bare the problems in that area.
|Europa League Group A table|
This was evident even before the departure of Virgil van Dijk to Southampton, with some observers identifying the loss of Jason Denayer as having had a greater impact in that regard.
With both gone, though, Deila has been unable to field, or perhaps even identify, a first-choice central defensive pairing, with new signing Jozo Simunovic only able to finish one match thus far.
Dedryck Boyata, at fault for the first of Molde's three goals, has much to do to convince supporters he can cut it, Tyler Blackett is a work in progress and Efe Ambrose has been found wanting on big occasions, not least when Celtic threw away a two-goal lead at home to Fenerbahce.
|Celtic lose in Molde|
|The Bhoys have kept one clean sheet in their last 18 Europa League games, conceding 32 goals in a run stretching back to December 2009.|
|Celtic have now let in two or more goals in each of their last five European games (including qualifiers).|
|There were 47 seconds between Kris Commons' goal for Celtic and Mohamed Elyounoussi's for Molde.|
But while the conceding of some goals has been down to individual error, a tactical set-up that commonly leads to Emilio Izaguirre and Mikael Lustig being caught too far up the park has to be questioned.
It is a ploy that, as assistant manager John Collins infamously observed, is rarely punished on the domestic front where Celtic will continue to dominate regardless of European performances.
A recurring theme
But time and again, Celtic have been punished for failing to adequately adapt those tactics to allow for a higher calibre of opposition.
Deila has to take responsibility for failing to learn from a weakness that has haunted them since the wretched displays against Legia Warsaw in last season's Champions League qualifiers.
As a team, Celtic have not performed to the required level when it has really mattered in Europe under Deila.
To the Legia debacle add Maribor, Malmo and now Molde. It was only three years ago that Neil Lennon's side were beating Barcelona in the Champions League group stage - a world away from the level at which this team is performing.
Some fans - principally the ones venting their anger at Celtic's chief executive Peter Lawwell on Thursday night - would argue this is a result of a gradual downsizing over that period, selling players and replacing them with inferior versions.
Celtic operate as a highly successful business but at some point - Thursday night being a prime example - that policy comes under scrutiny as a club with a much smaller budget is able to brush the Scottish champions aside.
But that brings the argument back to how the players are coached and then set up as a unit, an area in which Deila has been found wanting if European results are anything to go by.
Lacking the Commons touch?
And then there is the matter of his management of personalities, which came into sharp focus as Kris Commons berated the decision to take him off on Thursday night.
Deila's relationship with Commons - a strong character and a powerful one within the dressing-room dynamic - has been the subject of debate almost since the Norwegian's arrival.
A belief that Commons is not Deila's type of player persists, but in recent weeks the forward has started the majority of matches as Celtic began to put together a run of form that took them to their familiar spot at the top of the Premiership.
Commons is a passionate man and perhaps his reaction to being substituted was nothing more than an example of a player's frustrations at being replaced - arguably unjustifiably.
But as Deila labelled that reaction as "unacceptable" and Commons was shepherded past reporters afterwards, speculation will resurface as to the management team's ability to command the respect of some of the club's senior players.
The best way to quell that will be to reaffirm their domestic dominance against Dundee United and Aberdeen in the league and Hearts in the League Cup before a revenge mission against Molde in a fortnight's time.
Anything less and Deila will be in for an even more uncomfortable ride than the turbulent plane journey back from Norway.