Neil Lennon conceded there needs to be a culture change at Celtic after his side suffered an "unacceptable" 4-1 defeat at home to Sparta Prague to leave their Europa League hopes hanging by a thread.
Lukas Julis scored a hat-trick as Sparta claimed a stunning away win that sent Lennon's side to the bottom of Group H with one point from three games.
The Czech Republic side had a clutch of players missing through injury and Covid-19 issues, including No 1 goalkeeper Milan Heca and teenage star Adam Hlozek, but it proved to be a chastening night for Lennon, whose side have now won just one of their last six games in all competitions.
"There needs to be a culture change within the club," Lennon said. "I need to look at the culture, get them more hungry, get them back to basics. All those things Sparta did.
"We have to be far more aggressive in our play. As the manager I have to accept responsibility but the players have to look at themselves too.
"It's an accumulation of things catching up. We need to show a bit more humility and get back on the training ground. It's a bitterly disappointing performance.
"We don't do the basics well enough. The first goal, it should never get past the first post. It's lackadaisical and we have to do better.
"The group is probably gone. We need to get some respect back with our next performance. I've been here a long time and that's a very poor performance from a Celtic team."
Celtic went into the game on a high after their impressive 2-0 William Hill Scottish Cup semi-final win over Aberdeen at Hampden on Sunday, which ended a rare four-game winless run.
Lennon kept an unchanged side but it was a somewhat calamitous night for the Scottish champions, who came unstuck in a tumultuous spell midway through the first half.
Sparta didn't look back after Julis' opener, and Lennon added: "It was poor. We started the game OK and then got really ragged with the ball. Our passing, technique. I keep saying if we don't defend set-pieces properly we'll get punished.
"There was a lack of hunger tonight and I don't know where it's come from because we've put in a really good performance on Sunday. I've defended the players up until now but that was unacceptable tonight.
"We go from top in the group last year against teams of a similar standing and now we're one point in nine. I don't know where that's gone, so we need to take a look at that.
"We picked the same team because of the performance in Lille. Nothing surprises you in football but that first-half performance was very poor. The defending for the third goal was very poor.
"This isn't the biggest challenge I've faced in my time at the club. It feels like it is because it's present. But it won't happen again, that's for sure."