Celtic lost 2-1 to Benfica in the city immortalised by the European Cup winning side of 1967, leaving the Hoops still poised on the brink of qualification for the last 16 of this year's tournament.
Goals by Ola John and Garay eclipsed a header by Giorgios Samaras to seal a win in Lisbon's Luz stadum, which means the Glasgow club still have it all to do in the final round of Champions League fixtures on Wednesday December 5.
They will entertain Spartak Moscow at Celtic Park, while Benfica travel to the Nou Camp to face Barcelona, who have already romped into the last 16.
Neil Lennon's men will regard their tie as the easier of the two since Spartak are already eliminated, but the Portugese are now second in Group G and need only to match Celtic's result on December 5 to qualify, given their better head-to-head record.
More than 5000 Celtic supporters inside the rain-soaked stadium did their best to lift their heroes, but Benfica created more clear-cut chances and goalkeeper Fraser Forster had to pull off a number of heroic saves to keep them in it.
Celtic's Victor Wanyama will be suspended for the Spartak match, having been booked to add to an earlier yellow card.
Lennon admitted after the match he was disappointed by the outcome, and his team's lack of composure at crucial moments.
He also revealed that skipper Scott Brown could now face surgery for his troubling hip problem. Brown had to be replaced again in the second half after collapsing to the turf.
He said: "I'm disappointed with the two goals we conceded, particularly the second one.
"I was disappointed with the lack of composure about us and the nervousness on the ball.
"However, we have one more crack at it and know what we have to do."
Lennon put the nervousness down to it being a young team and he admitted the loss of victor Wanyama for the crucial game against Spartak through suspension will be keenly felt.
"It will be a huge loss because he has been magnificent, he will be a big miss.
"I thought some decisions really went against us sometimes but hopefully we will be getting some players back for Spartak. A few are running on empty."
Samaras was staying positive: "For 90 minutes we were a little bit nervous but it is finished.
"The team effort and the really hard work we had...we must stay positive, we can still go through.
"I am not really happy. I reckon we played really well today, the game was really close, we lost the game in the details. I am just looking forward to the next game."
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