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."
Bayern Munich are kings of Germany and hope to be crowned kings of Europe at Wembley later this month!
The Juventus jersey for the 2013/14 was released earlier this week. Be the first to wear the colors of the Old Lady.
Ajax has released their away soccer jersey for the 2013/14 season. Get in the Gear.
The push for more color on their cleats, adidas has added the infrared colorway to the adiZero f50 Line.
Borussia Dortmund hears Wembley Calling and are headed to London for the Champions League final.
One of the top soccer cleats for control the Nike CTR360 will not let you down on the field in tight spaces.
The adidas Nitrocharge is the most talked about new cleat in years. It was released on May 15 and will take your pitch by storm.
Chelsea won the Europa League on Wednesday. The Blues defeated Benfica 2-1 on injury time goal from Ivanovic.
The Liverpool 2013/14 soccer jersey is available for pre-order. The Reds are eager to compete for silver next season.
The new PUMA evoSPEED 1.2 FG. A great new soccer cleat with classic look and first time for PUMA to be on the cleat.