On a bleak night, there was a moment of dark comedy when Brendan Rodgers sat down to assess the unmerciful hiding that had just been inflicted on his team.

"We weren't quite at our best," said the Celtic manager in the wake of the deluge that washed over his players.

He said it without fear of contradiction. After watching his team lose 7-1 while committing so many errors that 10-1 wouldn't have flattered Borussia Dortmund, 'not quite at their best' was a fairly unarguable take.

Dortmund preyed on Celtic's naivety and rendered them quivering wrecks inside half an hour. It was brutal. Their end was swift and horrible.

The errors that the spooked visitors made were jaw-dropping. The mere sight of a yellow and black jersey turned their composure to mush.

Some of the goals were outstanding finishes, lustily banged home by players who couldn't believe their luck.

Julian Brandt found as much space as he liked in midfield and nobody thought it sensible to do something about it. But Celtic don't really have a player who is made that way, a destroyer, a guy who can sense danger, a character who can - Lord forbid - let Brandt know he's there.

In a word, Celtic were soft.

Many of the goals had their origins in players losing the plot in possession. Daizen Maeda did it; Alastair Johnston did it; Auston Trusty did it. Three Celtic players did it within seconds of each other in the lead-up to the seventh goal.

The "acid test" Rodgers called this. "For us, it's looking to bring our game to the next level," he said on the eve of the match.

Of course, he didn't say which direction he was thinking of when talking about the next level. Up or down?

Dortmund might have noted Rodgers' fighting talk about how great a place the team was in (he didn't mean Dortmund) and how he knew that they had the mentality to "hurt teams".

Based on weekly routings of clubs with a tiny percentage of their budget. Flimsy evidence that they chose to interpret as compelling. Again.

This was a monumental humiliation for Celtic, a team that once again fell into the trap of believing that just because you can play freewheeling football against St Johnstone in Perth on a Saturday means that you can try to do the same against Borussia in Dortmund on a Tuesday.

Pragmatism? Closing the space? Staying in the fight? Keeping it tight and compact in the face of Dortmund's obvious pace and danger? No, no. They set up like they set up against St Johnstone and Falkirk and Hibs and Rangers.

They felt they could go toe-to-toe with Dortmund because that's what they do in Scotland and opponents fall at their dancing feet. They were pretty bullish about their readiness to transfer domestic superiority into the unforgiving fields of Europe. They're nowhere near.

Until Rodgers introduces some overdue realism when playing some of Europe's best, then this is likely to continue. There'll be another shellacking down the line if he doesn't change course.

Celtic are repeat offenders under their manager. That's a second 7-1 Champions League defeat he has suffered at the club. That's on top of a 5-0, a 6-0 and a 7-0.

Most of the biggest beatings in Celtic's European history have Rodgers’ name beside them. He's been a brilliant domestic manager, but Europe has been painful.

We saw again Celtic's habit of conceding goals in clusters. They shipped three in quick order in Dortmund and the game was done.

Last season they conceded three times in 16 minutes in the 6-0 loss to Atletico Madrid and twice in three minutes against Lazio in Rome.

This goes back a while. In the Europa League in the last season of Rodgers' first spell in 2018-19, they lost two in six minutes against Salzburg, two in four minutes against Leipzig, and two in seven minutes against Valencia.

In other Champions League games under Rodgers, they conceded two in two and another two in six on the same night against PSG. In the return game, they lost two in four and another two in six.

They shipped three goals in 10 against Barcelona in 2016 in that 7-0 defeat.

What happened to them in Dortmund was a shock, but it shouldn't have been.

It's just a continuation of a theme involving a manager who has been singularly unable to achieve even the modest goal of making his team resilient in the face of superior opposition. Too often they go down in flames.

Later on Tuesday, Rodgers was asked if he would do anything different in the way of formation. "Not really," he replied.

Their next game is away to Atalanta, who beat a mighty Bayer Leverkusen 3-0 in last season's Europa League final and who started their Champions League campaign with a 0-0 draw at home to Arsenal.

That one has menace written all over it if Celtic sleep-walk into it in the way they appear to have sleep-walked their way to Dortmund.

In the preamble to Tuesday, Rodgers outlined his goals for the night. "I'm not looking for perfection," he said, which was just as well.

What he got had a wearying familiarity to it. Pride before a fall and all of that.

You'd say he needs to learn lessons about how he sets up his team for these tough assignments in Europe, but those things were said in his first spell, too.

History is repeating itself in more ways than one.