Let me open by borrowing a phrase from a former heavyweight champion of the world: “Everyone has a plan until they get punched in the face.”

Mike Tyson was talking about the rigours of the boxing ring. Right now, his famous quote equally applies to Scotland’s football champions.

Brendan Rodgers, remember, has never been better prepared for a tilt at the Champions League. His own words. So how on earth did his side, freshly bolstered by a recent transfer spend of over £30m, concede three goals inside half-an-hour, five goals before half-time and seven by the time the sorry, non-contest in Dortmund was put out of its misery?

That Celtic lost for the first time this season, at the home of last term’s European runners-up, is no cause for shame. The scoreline, however, very much is. As is the manner of the capitulation.

After a 21-match unbeaten run stretching back to early March – a ruthless streak featuring 20 wins – Rodgers’ men were always likely to come undone in front of the famous ‘yellow wall’.

Curiously, though, the last time the champions of Scotland went to Dortmund, the result was altogether different. Rangers ran out 4-2 winners against a team featuring Jude Bellingham, Marco Reus and Manuel Akanji.

Which begs the question: why couldn’t Celtic at least be competitive in the same arena?

It’s the one thing Rodgers was at pains to point out before a Champions League ball was kicked this time around. His team is not here to participate in the tournament but to compete. They singularly failed to do that against a side currently fifth in the Bundesliga.

No panic buttons need be pressed, of course. After two games in the new format, one at home and one away, Celtic are sitting on three points. Exactly where they thought they would be.

The concern for Rodgers is that this was yet another huge helping of humble pie served up at European football’s top table. He must have indigestion by now.

It was the third time as Celtic manager he’s watched his side ship seven goals in Europe. Granted, two of those were against Barcelona and Paris Saint Germain. The big worry, however, is that this is still happening eight years after Messi, Neymar and Suarez crushed De Vries, Toure and Gamboa in the Nou Camp.

Lessons very clearly are not being learned, meaning the same patterns are being repeated. What’s that definition of madness again?

The impressive victory over the champions of Slovakia on matchday one shows Celtic can deal with a certain level of European opponent. The battering by Borussia suggests compromise must be reached if Celtic are to emerge with any semblance of pride, never mind points, from the more daunting arenas.

What do you do if the other team has better players than you?

A question very familiar to every other manager in Scotland, Rodgers will surely now have to give it proper consideration before his next venture overseas to face last season’s Europa League winners, Atalanta.

Not one of the giants of the game, but a team just as capable of embarrassing Celtic yet again if the Scottish champions can’t find a formula to allow them to compete with sides who are more savvy at this level and, frankly, technically superior.

In short, Celtic have to become something entirely new on foreign soil. Difficult to beat.

They won’t get any kind of preparation for that at home, they’ll return to their domestic comfort zone without too many problems. History tells you that.

What the recent record books also show is that the fearsome European beatings will continue against the better-ranked sides unless Celtic can find a way to contain and counter-punch.

Like a boxer taking too many blows to the face, it’s surely now time for Rodgers to put up his guard.