The towering Norwegian was lured to Glasgow from IK Start but made his first-team debut under Brendan Rodgers, right, in the 3-0 victory over Lincoln Red Imps 19 months ago.
However, since then his character has been forged by triumphing (not always convincingly) over adversity. During that time he has endured disappointments but continued to bounce back from them.
Ajer did not feature for Celtic again last season following his cameo against the part-timers of Gibraltar. Instead, he spent the second half of that campaign on loan at Kilmarnock, who secured their Premiership status with only two fixtures remaining.
Injuries to others this term meant he started the second leg of the Champions League play-off against Astana, with Celtic protecting a 5-0 lead. However, dilatory defending from a makeshift rearguard saw the hosts race to a 4-1 lead midway through the second half.
Fortunately for Ajer, Celtic notched two late goals to ensure qualification but it was an experience which would have scarred many other youngsters.
“It was quite a frantic night,” he said. “After that, I learned so much by looking at the clips with the staff and the gaffer. I saw what I could have done better individually and what we could have done better together as a team.
“I think we played on a really good day for them as well. You have to say that they played a really good game – maybe because they weren’t under so much pressure after the result at Celtic Park.
“They played very well. We conceded a few bad goals from our point of view but it was a night where we still went through and that was the most important thing for me.
“It was still a good night for me, learning-wise, and I feel in a much better place now.”
Ajer points out that his time battling against the drop at Rugby Park under Lee Clark and Lee McCulloch – when more games were lost than won – also served to accelerate his development.
“Playing for Kilmarnock allowed me to play really important games,” he said. “We were fighting against relegation so every one was crucial.
“But here there’s an expectation to win. That’s really good for your development. When you play for such a big club, you demand a lot from yourself as well. Everything is bigger here in terms of the pressure and the number of fans; it’s just on a bigger scale.”
Having sat out the Champions League group games, Ajer has subsequently forced his way into Rodgers’ starting XI and he has no fears about facing the Russians.
“I’ve not even looked at them online yet,” he said. “I’m taking one game at a time; I’m still young and know that I need to perform in every single game.
“There are so many games at the moment so I’ve had to adapt to playing two games a week. I’m really feeling that my fitness is on top just now and I’m ready for each match.
“In terms of nutrition and food, I feel I’ve done everything I possibly can. But each week is different and you need to be looking at how much you can train, relax and recover before the next game.”
Like his defensive team-mates, Ajer has been encouraged by Rodgers to play his part in the team’s passing game, although that has, on occasion – notably in the 0-0 draw with Rangers at Celtic Park in December, when conceding possession almost led to a goal for Graeme Murty’s side – caused problems.
“When you play under the gaffer here he has an expectation of how he wants you to play,” he said. “I just try to play to my strengths. Sometimes you lose the ball but that’s football. As a centre half, obviously you don’t want to lose the ball too often so I’m working on that.
“It helps that I was a midfielder before. I feel that, when I have the ball and see opportunities to go forward, I can take them. Playing centre-mid before helps me now.”