Tony Mowbray was handed a harsh dose of reality in his first competitive game as Celtic boss as his side crashed 1-0 to Dinamo Moscow in the first leg of their Champions League third qualifying round clash at Parkhead.
Alexander Kokorin's seventh-minute strike gave the Russian club a clear advantage ahead of next week's return game in Moscow.
The Dinamo frontman was sharpest to Alexander Kerzhakov's cross from the left to slip the ball past Hoops' keeper Artur Boruc and, while the home side battled for the rest of the game and created several decent chances, it was to no avail.
Celtic's pre-season form, which saw them unbeaten in four games without the loss of a goal, had offered false optimism to the Hoops' fans who now face the very real prospect of dropping down to the less prosperous and less prestigious Europa League.
It took only seven minutes for the Parkhead supporters and Mowbray to be handed a reality check by the visitors, who, with 15 Russian league games under their belt, looked sharp from the kick-off.
From Kerzhakov's cross, Kokorin nipped in to toe-poke between Artur Boruc's legs from around six yards to silence Celtic Park.
The Parkhead side struggled to cope with the sheer speed of Dinamo and, in the 14th minute, Dinamo midfielder Dmitry Kombarov all too easily whipped a terrific ball in from the left only to see his twin brother Kirill Kombarov's volley slip just inches past Boruc's right-hand post.
Celtic's first real effort on goal came midway through the first half but Marc-Antoine Fortune's turn and drive from 10 yards was easily saved by Dinamo keeper Vladimir Denisov.
Mowbray's men continued to battle although Fortune was not having the best of luck. His back-post volley, after Aiden McGeady's cross from the right had ended up at his feet, was easily saved by Denisov.
However, just 60 seconds later, in the 27th minute, the former Nancy striker missed a sitter from three yards when Shaun Maloney set him up, Fortune failing to hit the target as he stretched at the far post.
It was a compelling, enthralling game. Dinamo's Australia midfielder Luke Wilkshire had another long-range effort on goal after the Celtic midfield temporarily went missing, seconds before Scott McDonald's point-blank drive at the other end was parried by Denisov.
The Glasgow giants finished the first half on top but Massimo Donati's hopeful long-range drive, which went yards over, was symptomatic of their threat.
Celtic came out after the break needing to score at least two to boost their chances in next week's second leg in Moscow.
And with little more than a minute played of a whirlwind restart, Dinamo stopper Denis Kolodin had to clear McDonald's header off the line after the Australia international had leapt at the back post to knock Maloney's cross past Denisov.
The Parkhead side had picked up the pace of the game but, in throwing players forward, they left themselves open to the counter although the visitors appeared happy to sit and hold their lead.
A blue defensive wall met the Celtic players as they pushed and probed and, in Kerzhakov, Dinamo had the perfect out-ball.
Mowbray replaced McDonald and Fortune with Chris Killen and Georgios Samaras on the hour mark in a move that signalled a hint of desperation and a change of tactics, to a more robust approach.
Danny Fox then replaced Donati to make his competitive Celtic debut after signing from Coventry at the weekend but the home fans were becoming increasingly irate as the second half unfolded.
The hosts pushed with less conviction with each passing minute, typified in the 74th minute when Fox headed high over from Andreas Hinkel's searching cross, before Killen headed wide from a Fox corner.
With five minutes remaining Boruc was called in to action to make a fine block from Dinamo substitute Fedor Smolov, on for Kokorin, seconds before Samaras headed in to the arms of Denisov.
When the final whistle sounded, it may just also have signalled the end of Celtic's Champions League hopes this season.