Their 25-man squad that was registered with Uefa in September for the group stage can have three new names added, provided three players are removed.
Left-sided defender Rami Gershon arrived at the Hoops' training camp in Marbella last night to begin his quest for a place in the squad along with midfielder Tom Rogic who is there on trial.
And with Mo Bangura now back in the fold following his loan spell at AIK, Celtic No.2 Johan Mjallby believes changes could be made ahead of next month's first leg.
He said: "We've 25 players in there, and he (Gershon) needs to prove he can bump one of them out of that squad.
"I've seen some of his games and he likes a tackle.
"Rami was doing really well in Belgium, but then he got an injury and was out for a while.
"If he's fit enough and gets confidence, he'll have a good chance to stay. Hopefully, he can hit the ground running.
"It's a good chance for him to prove himself and prove to everyone he's good enough to get a longer contract."
Provided he shows he is ready to play, Gershon, who is on loan until the end of the season from Standard Liege, will get his first chance to stake his claim in the friendly against Steaua Bucharest which will take place here next Tuesday evening.
But manager Neil Lennon – who only joined up with his players late last night – is expected to be absent from this match as he heads back to continue his player scouting.
Despite reports the Hoops were about to make a bid for Olympiakos striker Konstantinos Mitroglou, sources at the club have said he is not on their wanted list.
And Nigerian international Juwon Oshaniwa, who is currently with the Super Eagles as they prepare for the African Cup of Nations, has claimed talks are ongoing over a possible move to Celtic.
Rogic, who is on trial from Australian side Central Coast Mariners, is one of the men who has been brought to his attention, and the 20-year-old attacking midfielder will be closely monitored during his six days with the squad.
If they like what they see, Celtic are ready to bid for the three-times-capped A League star.
Unfortunately, his club will not give permission for Rogic to play in Tuesday's game against the Romanians.
Mjallby said: "Training is always good, but you prefer to see them in games when the atmosphere is a bit different and players are impressing a bit more.
"I've heard very good reports and the sooner we see him, the better."