The Celtic defender gave away a clear penalty that put the Hoops behind on the night.
The Norwegian international hauled down M'Baye Niang in the first half, with the referee not hesitating before pointing to the spot.
Replays showed that the young defender had a clear hold of not only Niang's shirt, but his arm too, also lunging in and tripping him.
It's an error that appears to have cost Celtic victory, with Ryan Christie netting a penalty of his own to equalise and secure a 1-1 result.
The Hoops could be wondering what might have been if not for the clumsy challenge.
Bonner was critical of Ajer, telling BBC Scotland (19/09 live match page, 18:46): "Kristoffer Ajer got too tight, went with the wrong leg, held his jersey and his hand.
"How can you do that when the referee is ten yards away from you? It was a stonewall penalty. A horrendous mistake from Ajer."
McFadden meanwhile added on Radio Scotland (19/09 live match page, 18:34): "Terrible defending from Kristoffer Ajer. The first time that Mbaye Niang has caused any kind of threat.
"Ajer just gets too tight. Be tight but don't dive in. He makes a terrible decision. You can't be defending like that, certainly against a player who has played at the level Niang has."
There's no doubt it was a horror moment from the 21-year-old.
Lunging tackles are something that he should try and eradicate from his game. He's shown in his early career he has a tendency to rush in and over-commit on challenges.
He's a far more robust defender on his feet shepherding opposition attackers. He'll be disappointed with his contribution tonight.
It is important to remember though he is still a very young player essentially learning his trade as a centre-back. As consistent as he's been over the last six months, moments like tonight's are still liable to happen.
Lennon is unlikely to judge him too harshly as a result. The Celtic boss has marked the player out as a real favourite of his since arriving for his second spell as manager.
Ajer will have better nights in a Celtic shirt and he's likely to show that in the rest of the group stage.
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