Neil Lennon threatened his players with bench duty in Wednesday's Champions League opener against Benfica after their defeat at St Johnstone.
Kris Commons drove the champions into the lead in the third minute at McDiarmid Park and it looked like it would be a comfortable win over the struggling Perth club.
However, Gregory Tade levelled in the 18th minute before Rowan Vine's first goal for the Saints with 10 minutes to go gave the home side a 2-1 victory -their first Scottish Premier League win of the season.
An angry Lennon, who revealed skipper Scott Brown will need to be assessed after picking up a hamstring problem, said: "There was a softness that I haven't seen and subconsciously it was with Wednesday night in mind.
"A lot of them had half an eye on Wednesday night and it is making me re-think my team in terms of their attitude.
"If they are being complacent or cruising through games they won't play, that's not what they are paid for. That performance was unacceptable.
"I am very disappointed. It was a poor performance from us. After getting off to a good start we let the game drift away from us.
"It was a needless performance, it was unacceptable. We could have been beaten by anybody today.
"We had so many stray passes, we played like strangers at times.
"We conceded no goals in three of the four Champions League qualifiers and we have conceded seven in five league games so there is definitely a mindset there that we need to change very quickly.
"They have a lot of good publicity but the game can bite you very quickly and it has bit them today and I hope they have been brought back down to earth with a crunch."
Celtic's group includes Barcelona and Spartak Moscow but defender Mikael Lustig insists the Parkhead men have a chance of making it into the knockout stages.
He said: "Barcelona are probably the favourites and it is hard to say after that.
"We are going to be really strong at Celtic Park where we have amazing fans so hopefully we will take points against every team there.
"It is going to be tough away but I think we have a really good chance.
"If we have a chance to go through we can't drop many points at home. The first match against Benfica is at home so we want to get points."