The 2-1 defeat last night means they must better Benfica's result on December 5, or drop down to the Europa League. That would still mean European football after Christmas for the first time since 2008.
But it is the draw for the knockout round of the Champions League that Celtic want. And they will go all out in their final match against Spartak to achieve the win which could still be enough to open the door. However, if Benfica can make it three consecutive wins in Group G when they go to the Nou Camp that night, there is nothing the Hoops can do to avoid finishing third in the table due to a poorer head-to-head with the Portuguese.
With Victor Wanyama suspended after picking up a third caution last night, and skipper Scott Brown back on the injured list after limping out of the tie, the odds are stacking up against Lennon's side.
But Joe Ledley – who revealed he is battling against a persistent groin injury –remains confident Celtic can still go through with Barcelona, even though for the first time, their fate is no longer in their own hands.
He said: "We defended brilliantly once again as a unit, but it was just a couple of seconds of switching off which allowed them to score the winner.
"We didn't pick up the second ball from the knock down.
"All we can do now is go out and get the three points against Spartak, then see what has happened in Barcelona.
"We have an advantage because we are at home
"Even though Barcelona have already won the group, they have a fantastic squad, no matter who they put out on the night and I am sure they will not be taking the game against Benfica lightly.
"But Spartak will have no pressure on them, also, so we know we face a difficult game."
Having passed up the first opportunity to book their place, Ledley and his team-mates are determined not to disappoint their fans and themselves again.
He said: "It would be very disappointing having got this far in the competition, if we did not manage to get through.
"But we have got one more chance.
"We are all disappointed Victor Wanyama won't be able to play because of his suspension.
"We need to put a good performance in for him against Spartak and, hopefully, qualify for him because he has been fantastic.
"I hope I'm ready, but I will have to wait to see what the physio says about my groin problem."