The pair, who are on international duty with Sweden and Bosnia & Herzegovina respectively, have both missed training this week.
Swedish media outlet Aftonbladet reported that right-back Lustig was forced to watch on in training after suffering a knock and could miss their qualifiers against Luxembourg and Holland, though it is suggested it may only have been a precaution.
The Bosnian FA earlier confirmed Simunovic would definitely miss the games against Belgium and Estonia after suffering a torn hamstring, leaving him a major doubt to make Celtic’s Champions League trip to face Bayern Munich.
Celtic manager Brendan Rodgers has already faced a defensive crisis this season, with Dedryck Boyata only recently returning to action following a knee injury.
Right-back Anthony Ralston has been struggling with a knee problem and Erik Sviatchenko returned to training last week following two months out, also with knee damage.