After nine wins from nine this season – including an impressive 5-1 destruction of Slovan Bratislava on matchday one – confidence was coursing through this Celtic side before what Brendan Rodgers described as an “acid test”.
Celtic have only ever won twice away from home in the Champions League, and never in 14 attempts on German soil. That never looked like ending here.
Drubbings at the hands of Europe’s big guns have become an unfortunate habit for Celtic in recent years, and this mauling will be filed alongside similar thrashings by the likes of the Real and Atletico Madrid, Barcelona and Paris St-Germain.
This Celtic side was supposed to be different, supposed to have found defensive solidity to go with the attacking potency they have displayed this season.
Missing their defensive lynchpin, Cameron Carter-Vickers, it was always going to be a huge task to contain a Dortmund team who looked like scoring every time they flooded forward.
Had it not been for some last-ditch interventions from Liam Scales and good saves from Kasper Schmeichel, Celtic’s record European defeat of 7-0 could have been beaten.
Sometimes you have to accept the opposition are just levels above, but Rodgers will be frustrated at the unforced errors that compounded Celtic’s problems.
This was always going to be Celtic’s toughest test and they came up desperately short. Now the challenge is to rediscover something much closer to their best for the remaining Champions League games.