STEVEN McLean may have blown the final whistle in Celtic’s game against Partick Thistle shortly after two o’clock at Parkhead on Saturday afternoon to ensure the home team survived a spirited fightback by their opponents and progressed to the quarter-finals.

But Kristoffer Ajer’s working day didn’t end there. The young centre half promptly returned home to watch a recording of the William Hill Scottish Cup fifth round tie. He pored over the footage of the game, pausing and rewinding sections he had done well in, freezing passages of play where he had been caught out of position, for several hours.

“I do that for every game,” said Ajer. “I go home and watch the full game again. That’s how I work. I look at the details, I look at the goals, I look at when I’m with the ball to find easier solutions next time. That doesn’t change from game to game.

“I sit until the evening. When you play I think that’s a big part of the game. You must watch it and learn from your mistakes and what you did well too. I go back and forth to learn things then I have a chat with the staff on the Monday when we meet again. It’s good and it helps me a lot.”

The teenager’s post-match routine shows an admirable desire to improve which, coupled with his mature performances, suggests he can establish himself as a regular in Brendan Rodgers’s team despite the intense competition there will be in his position when the squad is fully fit.

However, the re-run of the 3-2 win won’t have made for particularly pleasant Saturday night viewing. Once again at the weekend Celtic struggled when their rivals summoned up the collective belief needed to take the game to them and applied pressure to their defence. Their failings at the back resurfaced and were exploited.

Kris Doolan and Conor Sammon both netted and Ryan Edwards was unlucky not to join them on the scoresheet in injury-time and force a replay. Only a vital saving tackle from Kieran Tierney on his goal line denied him.

“It was a long game,” said Ajer. “It was a hectic game too. Thistle pressed us well throughout the whole game. They did really well to come back and never gave us a second on the ball. They played really well.

“Whenever anyone plays against Celtic they fight for their lives and we need to do the same. Every time you play in this shirt it will be a difficult game and you just have to accept that. We defend as a team every time and we know in this league they can punish you for mistakes. We don’t want to make mistakes.”

You would imagine Zenit St Petersburg, the expensively-assembled Russian side the Scottish champions will take on in the first leg of their Europa League last 32 double header in Glasgow on Thursday night, will have the confidence to attack. With Sebastien Driussi, Aleksandr Kokorin, Dmitriy Poloz and Emiliano Rigoni at their disposal, they certainly have players who can hurt their hosts.

Celtic will have to do far better at the back if they are to record a result which will give them a chance of progressing. Following the defeats to Paris Saint-Germain, Bayern Munich, Hearts and Kilmarnock and their latest display against Partick Thistle many of their supporters will fear the worst.

Ajer is , with Dedryck Boyata and Marvin Compper both out injured, set to start against Roberto Mancini’s men. He is a safer option than Nir Bitton or Mikael Lustig. Having featured against Rosenborg and Astana in Champions League qualifying and Rangers in the Ladbrokes Premiership this season and fared well he is confident he will cope with the occasion.

“If I get the chance on Thursday I will be ready,” he said. “I feel when I play for Celtic here I’m 100 percent focussed in every single game. For me, being 19 years old, every single game is big. It won’t make a change who we play against or what day it is. I’ll always try to do my best.”

Much of Celtic’s play going forward on Saturday, particularly that of James Forrest, who scored the first hat-trick of his professional career, was impressive, but the ease which teams are having scoring against them at the moment is complicating their attempts to win games greatly.

Partick can, despite being knocked out of the Scottish Cup, take heart from their performance against Celtic. Edwards is hopeful his side can finish the season strongly once again and move away from the relegation and play-off spots at the bottom of the Premiership table.

“We don’t seem to start great, but then it comes to January and we seem to turn it around,” he said. It’s happening again. I don’t know why that is. We don’t want to be in the position we're in and it’s not planned. We want to keep progressing. We have three massive games coming up starting next week against Dundee.”