Celtic manager Ronny Deila felt his players "lacked fight" in their 1-0 home defeat to St Johnstone.
Celtic saw a run of 10 domestic victories come to a halt after a stunning strike from Danny Swanson earned Ulsterman Tommy Wright's Saints a 1-0 win at Parkhead.
The match followed two big games in the previous six days when Celtic lost to Inter Milan before beating Scottish Premiership title challengers Aberdeen 4-0.
But Deila, who lost Adam Matthews to an ankle injury at half-time, said: "It's not good to find excuses. The performance was not good enough, but it has been a very hard programme for us.
"We have had some very important matches and today we got a big, big setback.
"We had a lack of fight, I think. We didn't fight enough. We started the game well enough, had some chances and didn't score.
"In the end we were too open and didn't press as a team. And offensively we got in the centre all the time. We have to get much more width in the team and get crosses, and not be easy to read."
St Johnstone manager Wright revealed Swanson had almost missed out through illness.
The on-loan Coventry midfielder marked his first Scottish Premiership start for Saints with a spectacular first-time strike from 25 yards into the top corner in the 54th minute.
He was taken off not long after, following a long night of illness.
"He was not feeling well this morning and he was doubtful for the game," Wright said.
"He has been unfortunate since he came in because he had only really played one game in a month before and picked up a wee injury after Queen of the South," added the former Northern Ireland skipper.
"We have been working hard on getting him match-fit. He is a quality footballer. He is very good technically and it's a tremendous finish."
Wright added: "We talked about having the balance not to be on the back foot all the time and to do that you have to keep the ball, and we did that particularly well, and we got a great goal."
Wright hailed the performance of Northern Ireland keeper Alan Mannus.
"I can't remember many clear-cut chances. There were a lot of chances and shots from outside the box which Alan has dealt with well, but overall no-one can suggest we didn't deserve the victory."
Meanwhile, more doubt has been cast on John Guidetti's Celtic future after he was linked with a summer return to Feyenoord.
The on-loan Man City striker missed out through injury.
"If he doesn't want to stay then we have to find someone else," Deila said.