The big Norwegian was tipped for a glamour move by interim Parkhead boss John Kennedy following Saturday's return to form against Aberdeen.
But his fellow countryman and former Hoops midfielder Riseth is not so sure.
Ajer - who enters the final year of his Celtic contract in the summer - has been watched closely by Serie A giants AC Milan over the past 12 months but Riseth fears the fallen Scottish champions' huge dip in form this season may tempt the Italian side to look elsewhere.
The 22-year-old has been a mainstay in a defence which has already shipped 43 league goals this term - compared to just 19 in the whole of last season - plus another 21 in just 10 European ties.
Those figures are likely to have been even worse had it not been for the former IK Start stopper holding things together as Celtic's season unravelled.
But Riseth reckons Europe's biggest clubs may be having second thoughts about the Norway international's true potential.
He said: "It's difficult to say where Kris will end up next year.
"Before this season he played really well and big clubs like Milan were looking at him. He was hot in the market.
"This season, though, he hasn't done quite so well.
"It's important to keep going well year after year if you want to reach a major club.
"If he is going to leave Celtic, of course he wants to find a new club - but maybe he's not going to find a top, top club because you have to be doing well for Celtic if that is to happen."
Ajer was one of the players reported to have angered former boss Neil Lennon earlier this season by expressing a desire to move ahead of the club's disappointing Champions League exit to Ferencvaros.
He was as guilty as any of the men in green and white for that humiliation against the Hungarian outfit as he appeared to have his mind elsewhere.
However, he was back to his best as he thwarted the Dons at the weekend with a string of big interventions.
That led Kennedy to heap praise on the centre-back, with the caretaker boss saying: "With his profile of speed, size, technique and ability, there's going to be interest in him."
But Riseth has warned Ajer he cannot allow his focus to shift again if he is to fulfil his ambitions of a move to one of Europe's major leagues.
Riseth – a League Cup winner with the Hoops in 2000 who was also capped 52 times by Norway – said: "Kris is a clever boy and and he will know he cannot be thinking about other teams if he does want to leave, because that will make him play bad.
"He has to keep thinking, 'I'm a Celtic player, I want to give everything for Celtic' – that's the way to make the big clubs take notice.
"First and foremost, he has to do a job for Celtic."