It was a poor performance across the board, and one that raised uncomfortable questions, particularly at the back.
The goalkeeping display did little to help matters, and with errors creeping in over a prolonged period, there will be genuine debate over whether a change is needed when Hibernian visit Parkhead on Sunday.
Off the pitch, the mood was understandably flat, but one player believes the schedule itself could work in Celtic’s favour.
Alex Oxlade-Chamberlain, who is not registered for European competition, watched the defeat from the stands alongside his fiancée Perrie Edwards, and feels the quick turnaround offers a chance to reset.
Speaking afterwards, Oxlade-Chamberlain admitted the situation is difficult but stressed the importance of moving on quickly.
He believes having another game so soon helps stop the disappointment lingering.
“I think that definitely helps when sometimes when you’re off the back of a disappointing result, you get the chance to quite quickly put it to bed,” he said to Celtic TV. “So then that’s the next challenge, to move forward and bounce back from that.
“I think it’s just being relentless. Every game that comes, you have to have the same mindset, the same energy levels. Different boys can come in and help out and then it’s your job to step up and try and make the difference.
“It can be hard at times when the games are coming thick and fast, but it can be good as well when you use them in the right way. Get momentum, get used to winning games, hopefully, and things start to flow a bit better. We’ve got a big enough squad to deal with that.”
Sunday now feels crucial. Not just for points, but for restoring confidence after a bruising European reality check.