PERHAPS Albert Einstein called it right when he suggested that the definition of insanity is doing the same thing over and over again and expecting a different result. Certainly, Celtic appear keen on putting the theory to the test.

If they have not entirely lost their sense of self in the process they have lost a sense of direction when it comes to competing on a European field. The Parkhead side have succeeded in going round in circles, embroiled in their own hapless groundhog day with defensive frailties the perennial narrative.

On Thursday night, the Krestovsky Stadium saw Celtic starring in a familiar movie, from the gifted early goal to the rabbit-in the-headlights freeze.

It is not the exit itself for the Parkhead side that represents the problem but rather the manner in which it unfolded. Eight months ago when Brendan Rodgers sought to lead Celtic back into Europe’s premier competition few would have found cause to grumble with a third-place finish in the group stage and a parachute into the Europa League.

And therein lies the rub for Celtic on and off the park. If they are competing at the stage of the tournament they should be and holding fort domestically, will there be a rush to bring out a chequebook and invest heavily in the playing side this summer?

Paul Lambert, club captain when Celtic last made it to a major European Cup final in 2003 under Martin O’Neill, appreciates the financial issues that clip the club’s wings when it comes to bringing players in but he believes the only way to improve as a team is to keep being exposed to top-level football.

“It is difficult,” he said. “You have some money there but it is small fry compared to what is going on elsewhere. You can sense the do it, do it, do it, approach from supporters desperate to go out and spend money on a marquee name but it isn’t as straightforward as that.

“Players want to play in Germany, in Spain, in England. They can command salary levels that are out of the reach of any Scottish club regardless of whether they are on a sound financial footing or not. But for me, the only way you get better at European football is by playing it again and again.

“And you have to be able to defend. Brendan will know that more than anyone.”

By the time the next rounds of European football get under way next month, only around 12 of the original 54 nations will have representatives in continental competition; only a handful will come from outside Europe’s top eight countries.

On paper, at least, there is an argument to be made for realistic ambitions being fulfilled at Celtic. The problem comes though with the bruising nature of what has been offered up along the way. Heavy, demoralising defeats and a campaign that lacked the thrill of previous exposure to European competition, has left Celtic feeling as though they have taken the money but not without cost.

The Parkhead side have a particular alchemy with European football but the manner in which the ceiling has closed in on them with every summer transfer window that passes due to inflated fees and salaries has cold shouldered them a little bit more.

The disparity of resources is inarguable but for Lambert there is more to it than that.

“Celtic had the balance of that tie after the first leg,” the Stoke manager said. “We would all concede that finances play a major part but to me that is a tie that Celtic could have won. It wasn’t entirely unrealistic to think they could have come out of that tie and gone on to the next round. They got an excellent win in the first leg and that was a dangerous result for Zenit. To me, there was a sense that the heavy defeats in the Champions League took their toll – it was as if there was a fear to go and try for a goal in case they left themselves exposed. Had they been able to go out and get that away goal the tie would have been heavily slanted in their favour.

“I am never one to compare eras because I think it is unfair but what I would say is that we had a team that were streetwise and strong enough to go out and have a sense of belief in a situation like that. We scored goals against top-level clubs – that [3-2] defeat in Turin [in 2001] still irks me to this day because we should have had a point against Juventus – and having that ability to go and score a goal alleviates pressure throughout the entire team.

“The balance just doesn’t seem right at the minute. You need to be able to stand strong in these games and play with confidence in yourself and in one another.”

For all that Celtic won’t break the bank this summer, a look at the back five that started the game in Russia on Thursday would suggest there may just be one survivor when Celtic next set foot in a European competition. Kieran Tierney, as well as being the posterboy for the club’s academy, is also one of the few who can be relied upon to hold his nerve when the pressure is on.

For now, though, the demands are more immediate.

A domestic campaign must come to the fore with Lambert stressing the importance of getting back on the bike, so to speak, at Pittodrie this afternoon.

“It’s the only way to get it out of your system,” said Lambert. “People look at it and can say what they like, but if you play for Celtic you don’t get it easy wherever you go. They’ll need to be back on it on Sunday and it is the best thing for you. Aberdeen will fancy their chances given the long flight and the poor performance but that is what it’s like at Celtic – you need to get your mind on the next one as soon as possible.”