Celtic's "lack of hunger and application" was "unacceptable" in their 4-1 home defeat by Sparta Prague, manager Neil Lennon has said.
Lennon admitted his side's hopes of qualifying from Europa League Group H have "probably" been ended.
And he vowed to change the "culture" among his players and that it could be achieved "very quickly".
"This is definitely a turning point - it won't happen again, that's for sure," Lennon said.
"We can't accept that and the players know that as well. I can't defend the players. I've tried to over the piece, but I can't defend them tonight."
It was the first time Celtic have lost three consecutive home games in 30 years - and the first time they have lost three consecutive home Europa League matches.
They now sit bottom of Group H, two points behind Sparta and six adrift of leaders Lille, who stunned AC Milan 3-0 in Italy.
Asked if he agreed it was an embarrassing defeat, Lennon said: "Yes, of course.
"Lack of application, lack of hunger and that's not us, so there has to be a culture change and I've got to change it quickly and the players need to change quickly as well because that's totally unacceptable for a club of our standing.
"It is an accumulation of things. We need to get back to showing a bit more humility and start working harder on the training ground."
'You can turn it around quickly'
The defeat by the Czech Republic league leaders came after a 2-0 win over Aberdeen in the delayed 2019-20 Scottish Cup semi-final and Lennon admitted "I don't know where" the poor performance came from.
"We had a good day on Sunday, had a really good week, but the hungrier team won and that's not us," he said. "We were well prepared for the game, but as the manager I have to take responsibility obviously, but the players have to look at themselves as individuals."
Lennon is hoping to lead his side to an unprecedented quadruple domestic treble by winning last season's Scottish Cup and end the current season with a historic 10th consecutive Scottish league title.
He thought the defeat by Sparta was "up there with one of the poorest" results during his two spells as Celtic manager but insisted: "I've had bigger challenges than this. It feels like a big challenge because it's present, but I've been over far worse than this.
"It is something you can turn around very quickly and it's something I'll be hellbent on doing in the next couple of days."