Evan Ferguson's first-half double saw Roma cruise to a 3-0 Europa League win at Celtic as Wilfried Nancy became the first Hoops manager to lose his first two games.
The Scottish Premiership champions were booed off at half-time as they trailed 3-0, while Celtic Park had almost emptied by the full-time whistle after a demoralising defeat left the hosts' hopes of staying in the competition hanging by a thread.
The new Celtic boss opted to stick with the same starting line-up that lost Sunday's top-of-the-table clash to Hearts at Parkhead, only for his side to fall behind after just six minutes as Liam Scales inadvertently headed a corner past Kasper Schmeichel.
It then went from bad to worse as Ferguson, told he "has to do better" by Roma boss Gian Piero Gasperini ahead of the game, struck twice before half-time as the visitors all but sealed a second win in Glasgow this season, having already seen off Rangers.
The home side did spurn a chance to get back into the match when Arne Engels saw his penalty hit the inside of the post in first-half stoppage time, with Nancy ringing the changes at the interval with a triple substitution.
One of those players introduced, Kelechi Iheanacho, was unlucky to see his strike ruled out for offside, as did Roma's Leon Bailey as the second period petered out, with Celtic now languishing down in 24th place in the standings and only clinging onto a play-off place on goal difference ahead of their final two games against Bologna and Utrecht.
All of which now makes Sunday's League Cup final with St Mirren a must win for Nancy.
Analysis by Your Site News' Adam Binnie at Celtic Park:
"Two defeats in four days as Wilfried Nancy's tough start to life as Celtic manager continues.
"His side were completely outclassed this evening. Far from ideal preparation ahead of Sunday's Scottish League Cup final against St Mirren."
Former Celtic goalkeeper Joe Hart on TNT Sports:
"It's an intelligent club. It's not just about being good at football here.
"The club have got to address this. They've got to unite the support, the players, the management.
"It feels like a soap opera. It's so financially stable, there's so much success.
"Last season was an excellent season but they've not addressed the problems, and the gap's widened.
"This is such a special football club when it's united. At the moment, they're drifting further and further apart.
"There needs to be a lot more understanding of what everyone's end goals are. Everyone in the stadium wants Celtic to be successful but you can only do it if they're together.
"It doesn't need to be like this. It comes right from the top. You can't rest on your previous success - football keeps moving.
"I'm so lucky that I got to spend three years here. At the moment, Wilfried Nancy won't know what it's all about."
Celtic boss Wilfried Nancy on TNT Sports:
"The reality is we are not able to cope with the intensity. And they tried to do well, but the first half was difficult. We were not able to come out of the pressure.
"We had a few moments where we unbalanced them, but we didn't keep the edge that we had. That's why it was difficult. And as soon as we had a bit of time to play forward, we didn't take the good decision, but it was not easy for my players.
"I really liked the second half, good reaction. We played with a little bit more proximity between players to escape the pressure. And we started a little bit more after that.
"I don't like to talk about luck, but the first goal that we conceded didn't help. To come from behind was not easy. So, listen, the attitude was good.
"The second half was better, but it was not enough. I cannot tell you that they didn't try. They tried. We should have been able to connect a little bit more, but we didn't do it. So, again, this is the situation. I'm not concerned because I really like the reaction.
"And for me, this is the most important. They deserve at least to score one goal to see after that if the dynamic can change.
"I saw already good things. After that, the result is not what we want, for sure. But I saw already good things. Today, I saw good things.
"I would like the results to change today. But, again, we play to win games, but the reality is this is not the situation. And we had also hit the post in the last game."
Celtic travel to Bologna on January 22 (5.45pm kick-off), followed by a home clash with Utrecht on January 29 (8pm kick-off) in their final two Europa League fixtures.
