Celtic's revival under Martin O’Neill came juddering to a halt in Jutland after they were thumped 3-1 by FC Midtjylland.
The Hoops’ hopes of prolonging their Europa League campaign into the knockout stages suffered a major blow as the Danes hammered in three goals in eight first-half minutes.
Teenage substitute Callum Osmand won a late penalty which Reo Hatate converted, but the 19-year-old was then taken off on a stretcher in tears after suffering what appeared to be a hamstring injury on a horrible evening for the Scottish champions.
O’Neill had seen how dangerous Midtjylland can be first hand just last month when they beat Ange Postecoglou’s Nottingham Forest, before the 73-year-old answered the call to return to Celtic after Brendan Rodgers’ shock resignation.
They had travelled to the Danish city of Herning with a sense of optimism too.
Back in the Hoops dug-out after 20 years, O’Neill had already led them to a morale-boosting 4-0 league win over Falkirk followed by Sunday’s rousing victory over Old Firm rivals Rangers to reach the Premier Sports Cup final.
But good vibes and a retro tracksuit can only take you so far and Celtic were ruthlessly taken apart at the MCH Arena as Midtjylland made it four wins out of four to stay top of the 36-team league.
Celtic actually had the first shot on goal after just 25 seconds, with Johnny Kenny - overlooked earlier on Thursday by Republic of Ireland manager Heimir Hallgrimsson for the World Cup qualifiers with Portugal and Hungary - firing straight at home keeper Elias Rafn Olafsson.
It was the Hoops only effort on target for the next 80 minutes. They were quickly under the pump, first when Callum McGregor was robbed in midfield and Dario Osorio fired narrowly over.
Auston Trusty then had to make a last-ditch block from Osorio, with Franculino Dju blasting the rebound over.
Former Rangers right-back Kevin Mbabu was denied twice by Kasper Schmeichel and Mads Bech Sorensen, the ex-Brentford defender, guided Osorio’s free-kick over the crossbar.
The hosts came even closer when former Bournemouth midfielder Philip Billing dummied a low cross for Cho Gue-sung, who forced a point-blank save from Schmeichel, with Anthony Ralston taking the rebound right in the face.
Schmeichel, back on home soil a day after his 39th birthday, was at full stretch to tip a 20-yard volley from Pedro Bravo wide.
But he could only hold back the tide for so long and the dam finally burst following the resulting corner.
Celtic cleared the initial cross, but Mikel Gogorza reached the byline again and his chip across goal was nodded in by Croatian defender Martin Erlic.
Less than two minutes later Midtjylland doubled their lead when Arne Engels and Ralston let Gogorza squirm in between them and whip a low drive past Schmeichel.
It was three by half-time from a Sorensen long throw, exciting Guinea-Bissau forward Dju collecting the second ball and curling his 18th goal of the season inside the far post.
Schmeichel protested that he had been unsighted by offside Midtjylland players but a quick VAR check concluded otherwise.
O’Neill threw on Osmand at half-time and the 19-year-old earned his side a penalty when he was brought down by Erlic.
But Hatate’s spot-kick could not mask what was a chastening evening for Celtic, while Osmand’s late injury dampened the mood even further.
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