Celtic released the popular and long-serving Swede this summer.
The Hoops released the long-serving Sweden international last month after seven years of service at Parkhead.
Lustig won eight Scottish Premiership titles in Glasgow and Celtic boss Neil Lennon wanted the club's hierarchy to extend his £24k-a-week contract [The Daily Star] but no agreement was struck.
Lennon also lost Jeremy Toljan and Cristian Gamboa in a treble blow of departing right-backs - and he is yet to sign a replacement for any of them.
But despite admitting that the 32-year-old leaves a big hole at Celtic on and off the field, Ajer, who stands at 6'5, is confident that the Northern Irish manager can find adequate replacements.
He told The Scottish Sun: "Mika was a fantastic player, and a great guy in the dressing room.
“It is going to be difficult to get a player in of his level but I’m quite sure the club are able to do that.”
This past weekend, Celtic fans at Paradise unfurled a banner in remembrance of the imposing Swede, who acknowledged the gesture on social media.
This was a massive player for Lennon, and Brendan Rodgers before him, and it's definitely going to be tricky to find a fitting replacement.
Have something to tell us about this article?