The Hoops boss must decide whether to stick with new Bhoy Jason Denayer or bring back Efe Ambrose to partner Virgil van Dijk.
A goalless draw against NK Maribor will be enough to put Celtic into Thursday's group stage draw alongside the likes of Barcelona, Real Madrid, Manchester City and Bayern Munich.
But for this to happen, Fortress Parkhead will have to be re-established - and that means making the defence watertight.
Both Barcelona and AC Milan emerged from Celtic Park with three points last term, while Juventus also won comfortably and Benfica held the Hoops to a draw the season before.
Maribor have arrived in Glasgow confident they can score and turn the tie around. But Denayer is equally determined to help the Hoops keep them out, and prove he has learned from his first taste of European action in the first leg in Slovenia.
That 1-1 draw left the 19-year-old unfulfilled because he felt culpable for the equaliser scored by Damjan Bohar.
"That was disappointing for me because it was my man who got through to score, so, of course, I was not happy about that," admitted the kid who will be making only his third appearance for the Hoops if selected tonight.
"My first Champions League match was a great experience, but I had hoped for a win.
"After the way we played, I don't think 1-1 was a good result.
"But it wasn't a bad result because it was an away game for us."
Now another completely new experience awaits Denayer, a major European night at Celtic Park, with Champions League music and expectation tingling every sense.
The loan defender from Manchester City can't wait to embrace it and said: "I know that at Celtic Park the atmosphere is amazing, even for league games.
"So for a Champions League night, it will be even more special.
"I believe it will be difficult for Maribor in that atmosphere."
The partnership with van Dijk could be a keystone in Celtic's European campaign this season.
But the fear is it could be dismantled before it has time to set if the Dutchman is sold.
Denayer is not only to be found side-by-side with Van Dijk on the field during games, but is also a constant companion away from the pitch.
It's a friendship and bond which has allowed them to fit together well and develop the kind of understanding essential for any successful partnership.
Denayer can only see it getting stronger, and said: "We're learning more about each other all the time.
"We've only played a couple of games together, but we're beginning to have a good relationship in the team. I think it's going to get better the more games we play."