Celtic saw a run of 10 domestic victories come to a halt after a stunning strike from Danny Swanson earned 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 as easy to read."
St Johnstone manager Tommy 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.
"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.
"I can't remember many clear-cut chances
There were a lot of chances and shots from outside the box which Alan (Mannus) has dealt 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 Manchester City striker missed the visit of Saints through injury but Deila appeared unimpressed with reports surrounding the player's future when quizzed by Dutch television journalists.
"If he wants to stay here then it's a possibility," Deila said
"I want him to stay if he wants to stay.
"If he doesn't want to stay then we have to find someone else
We can't force anyone to stay here
There are a lot of players who want to play here."
When asked why the Swede had not been starting regularly, Deila said: "That's an easy question to answer, because he is not good enough
"You have to be good enough if you are going to play every game
Today he was injured."
Source : PA
Source: PA