IT wasn’t so long ago that David Turnbull was the darling of the Celtic support. The diamond in the rough of a nightmare season in which the 10-in-a-row dream may have died, but a star appeared to have been born.

Now though, his impressive debut campaign seems to have been forgotten in the wake of his calamitous error for Bayer Leverkusen’s opener in the Europa League thumping of Thursday night, and amid a growing narrative that the 22-year-old lacks the minerals, the physicality and the fitness to be effective for Celtic in their biggest games.

His manager, Ange Postecoglou, is certainly no subscriber to such theories, but he does concede that his mistake against the Germans in midweek now poses a huge question of his character and mental resiliency.

He has now challenged Turnbull to show that he can come through this sticky spell in his Celtic career, and show the doubters that not only can he learn from such a chastening experience, but that he has what it takes to assume the responsibility of being one of his team’s main creative outlets.

“It is a test and there is no place to hide on a football field,” Postecoglou said.

“Yeah mistakes are going to happen but the important thing is what is your reaction? What is your reaction within the team context?

“It’s not an individual thing, when a player makes a mistake the whole team pays the price. So it’s not about you trying to redeem yourself for something you’ve done, it’s about the team trying to redeem themselves and what your role is within that.

“Dave is still a young guy and that is part of his development if he wants to be a top footballer. He needs to understand that we can avert those mistakes and that it can’t be an individual response.”

As Postecoglou touches on, the reaction he is looking for is a collective one as he prepares to take his team to Pittodrie for the lunchtime kick-off against Aberdeen tomorrow. Particularly when it comes to taking some of the numerous chances they are creating and currently squandering.

“A reaction in terms of people taking personal responsibility out there for our performances,” he explained.

“Wanting to be the person who takes responsibility. Our football has been good. We are creating chances we are not scoring. You can’t keep creating nine or 10 chances a game and come away with one goal or no goals at all. Somebody out there has got to take it upon themselves to say, ‘I’ll break that.’

“We are conceding goals through individual errors that, again, are easily avoidable if people are disciplined and understand the consequences of their actions rather than just going through games and accepting mistakes. You can actually be stronger mentally to avoid that happening.

“They need to take responsibility. Take nothing away, Leverkusen are a good team. They can beat anyone on their night. But that game shouldn’t have finished 4-0.

“It was self-inflicted rather than what they did.”

Given the recent derailment of the Angeball bandwagon then, there would be no time like the present for Postecoglou’s side to get back on track by finally registering their first domestic away win of the season.

“Absolutely, that’s our challenge now,” he said. “It’s going to be a tough game at Pittodrie. It always is irrespective of the form of both sides.

“It’s a chance to show the character we are going to need moving forward. The football is one part of it. That’s developing and will get better, no doubt. But you have to have the character within that.

“You have to have mental toughness and inner toughness to want to be successful.”

Postecoglou, as he has been keen to point out this week, doesn’t need any reminders of what is at stake should he be unable to arrest this current downturn in results quickly, but he is insistent that he is still not only up to that challenge, but actively enjoying it.

“That’s kinda been my history wherever I’ve been the starts were always difficult, always challenging,” he said.

“There were always people who were going to be sceptical about it in and outside the dressing room. I enjoy the bit where it’s up to me to make people believe in what we’re doing and it starts with the players. I’ve got to get them to believe in the journey we’re on.

“There is nothing wrong with adversity. I grew up understanding that adversity is what makes you strong and you’ve got to go through it. I can’t shield the players from it, I can’t shield the whole group from the adversity we are going through because I think if you can push through it you’ll become stronger out of it.

“It’s easy for me to say it and the proof will be in the pudding. Over time I am happy to be judged. But this has been consistent throughout my whole career. Wherever I’ve gone it’s taken a certain amount of time for things to kick in.

“There has definitely been more challenges because of the injuries and the disruption we’ve had. I felt we could have been further along but that doesn’t mean I’m going to shy away from it or that I have lost any resolve in what I’m doing.

“It’s up to me to show the way forward. That’s why I’m here and I will. You start off with just a few but all I need is a few to keep us moving forward, and the rest will buy in.”