Celtic's hopes of a European run suffered a major blow with a 3-1 home defeat against Norwegian champions Bodo/Glimt.
The visitors' league season does not kick off until April but it was Celtic who looked out of sorts in the first leg of the Europa Conference League knockout round play-off.
Goals early in each half from Runar Espejord and Amahl Pellegrino put the visitors in command and Celtic conceded two minutes after Daizen Maeda had headed a 79th-minute lifeline.
Hugo Vetlesen's deflected effort gives Celtic a mountain to climb when they head to the Arctic Circle next week.
Celtic came up against opponents who arrived in Glasgow ready to play their hosts at their own game. Like Ange Postecoglou, visiting boss Kjetil Knutsen started with three forwards supported by two attacking midfielders while his team were prepared to build from the back and press high up the park.
The visitors had not played a competitive game since December 12 and had lost players to the likes of Krasnodar, Lens, Hertha Berlin and Schalke since clinching their second consecutive Norwegian title.
However, they made a flying start and Joe Hart had already made saves before Espejord, a new signing from Heerenveen, slotted home inside seven minutes after some sharp passing around the edge of the Celtic box. Carl Starfelt could not get close enough to make the challenge as the forward netted from 12 yards.
Maeda had Celtic's best two openings of the first half, just either side of the goal. The Japanese centre-forward opted to pass the first time when a shot looked on and he could not pick out Liel Abada with his square ball. Maeda mis-kicked from his next opportunity.
The visiting goalkeeper made simple saves from Jota's speculative effort and Abada's weak strike but Celtic were struggling to create clear-cut chances. Nothing was coming off for attacking midfielders Tom Rogic and Matt O'Riley while Maeda was not linking play.
The first half could have got worse for Celtic when the ball fell for the unmarked Vetlesen in the home box but the midfielder sliced wide.
Celtic came out after the break with a bit more impetus. Jota and Callum McGregor shot over from 20 yards and the latter played Maeda in behind but the forward shot against goalkeeper Nikita Haikin.
The visitors doubled their lead in the 55th minute following some poor Celtic defending. Ola Solbakken skinned Greg Taylor and delivered a low cross. Espejord's flick left Josip Juranovic out of position and Pellegrino had time to stab past Hart from six yards.
Abada soon headed over from a good chance and Jota forced an acrobatic save but the visitors showed another side to their game with an organised defensive shape.
Celtic Park erupted with hope when Maeda headed home Juranovic's near-post cross but the visitors regrouped with a post-goal huddle and soon restored their two-goal cushion when Vetlesen's 25-yard strike hit off McGregor and looped over Hart.
Substitutes James Forrest and Reo Hatate could not make the most of late chances to reduce the deficit.
