THE absence of both Kristoffer Ajer and Dedryck Boyata was certainly, as Brendan Rodgers was at pains to point out in the immediate aftermath of their 2-1 defeat to AEK Athens, keenly felt by Celtic in Greece on Tuesday night.

But it is impossible to state the Scottish champions wouldn’t have conceded such soft goals in the Olympic Stadium had they been able to field Ajer, who was suspended, and Boyata, who has disgracefully downed tools in a bid to engineer a lucrative move to another club.

Celtic’s limitations at the back have, even with Ajer and Boyata available, been an issue for them in Europe for some time and should have been addressed long ago.

Where the responsibility for that costly weakness – missing out on a place in the Champions League group stages for the third season in a row will deprive the Glasgow club of in the region of £40 million in revenue – lies is a moot point.

Brendan Rodgers publicly bemoaned not being able to strengthen in certain areas after the double treble winners inexplicably missed out on John McGinn of Hibernian last week.

Failing to secure the services of the Scotland internationalist, a Celtic supporter who would have been an ideal long-term replacement for Scott Brown, certainly wasn't chief executive Peter Lawwell’s finest hour.

But who is to blame for the fact that, nearly two years after they shipped seven goals in a record European defeat to Barcelona over in the Nou Camp, this Celtic side still often looks, as they say in the East End, unable to keep weans out a close in continental competition?

Lawwell? Major shareholder Dermot Desmond? Head of Recruitment Lee Congerton? Or is it Rodgers himself?

Rodgers has spent comfortably over £20 million in the five transfer windows he has had since being appointed manager. Now, that is buttons in today’s inflated market. But is it really unreasonable to expect his team to have a more reliable backline with that outlay?

The only central defenders who have been brought on board during his tenure have been Marvin Compper, the German who was hardly been sighted since, and Jack Hendry, a 22-year-old who had, for all his potential, played fewer than 30 top flight games in this country. Couldn’t more have been done?

Wouldn't, too, holding onto Erik Sviatchenko, despite his limitations, a little longer have been prudent? Many feel the Danish internationalist was discarded too readily.

Rodgers has bristled at persistent questions about whether he will splash out big money on an individual who can occupy what key position.

He has, rightly, pointed out that the kind of money he has at his disposal wouldn’t get him the sort of commanding, cultured and ball-playing talisman that supporters crave.

The Northern Irishman, it shouldn’t be forgotten amid the fallout to the wretched Athens defeat, has also shown he can work well with and improve centre backs.

Norwegian youngster Ajer has, like so many, flourished under him. Boyata was nothing short of a joke figure and looked to be on his way out of Celtic, not for a £10 million fee either, when Rodgers came on board. This summer he started four times for Belgium in the World Cup finals. The chances are Hendry will prosper in a similar manner too. But he is very much one for the future.

But a more pragmatic approach, an increased emphasis on not getting beaten, a keener understanding of what can reasonably be expected, would help Celtic to avoid similar failures in Europe going forward as much as any expensive signing.