The Celtic defensive trio will likely be missing for the upcoming clash against Hamilton.
The paper say that Simunovic is set to undergo more checks on his knee after missing matches prior to the international break.
It means he will not only be out of the trip to Hamilton, but also potentially the Rennes trip next Thursday.
Bitton meanwhile is still struggling after being forced off against Rangers in the recent derby match at Ibrox. He's expected to be out for another couple of weeks.
Norwegian prospect Ajer is also out of action, but the report states he is hoping to return for the UEFA Europa League opener in France.
The news comes after winger Mikey Johnston confirmed he won't be able to participate at New Douglas Park this weekend.
What are Lennon's options now?
With Christopher Jullien still available, the Celtic boss isn't completely bereft of options in the centre of defence.
Hatem Abd Elhamed, signed to play right-back, also has a history of featuring at centre-back for previous clubs, so he is the likely choice stand in for the trio.
That means that Moritz Bauer could well make his first start for the club after making a substitute appearance in the recent derby clash.
It's not an ideal situation for Celtic clearly, but the club will be glad they now made late moves in the transfer window, else they could have had a real crisis on their hands.
With Ajer returning shortly too, the Hoops should be in decent shape heading off on their European adventure.
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