But despite the 6-1 mauling they took in the Nou Camp, the Hoops boss retains faith that this group can compete at Champions League level.
Lennon does admit he will have to strengthen further and go again, but understands the club's policy of cautious investment until group stage football is secured.
It is the first time a Celtic side has ever shipped six goals in a European game, and the result equalled the biggest defeat they have suffered in Euro action.
However, Lennon remains defiant, though he pulled no punches when assessing how his Bhoys sunk without trace.
He said: "Barcelona were fantastic, but we were very poor. From some of the players, that was an unacceptable performance and was nowhere near the performances they have put in previously against Barcelona.
"There was no bravery on the ball. We started off nervously and it looked like our heads went down at 2-0. We changed it to be a little bit more defensive, but that didn't stop the blood flowing.
"We were weak, and that's not been like us. That's something I have to address.
"There were only two or three players - Van Dijk, Matthews and Lustig - who really showed anything they are capable of.
"I excuse the goalkeeper in that, as well. But the rest of the team were very, very poor and well below the standards they've set themselves."
Lennon explained Charlie Mulgrew had been left out of the starting line-up because of a groin problem, while Kris Commons was dropped to the bench because he wanted to utilise the pace of Derk Boerrigter and Georgios Samaras out wide.
He had asked his players to give him a performance and a result to complete the group, but conceded painfully: "I got neither. I'm angry and disappointed, but it's something I have to get over very, very quickly. It's very tough to take, We were playing a great side, but we're better than that."
Lennon felt things were not right even before kick-off, sensing the players were too quiet.
He added: "We didn't play anywhere near our strengths. I wouldn't say some players gave up, but it certainly looked that way at times.
"I'd never accuse a player of doing that, but there was a lot of half-hearted performances. As a manager, that's the most disappointing aspect."
Lennon now wants to put the horror show behind him and come again, and that will require recruiting players of better quality.
But he denies it is becoming impossible to be competitive against Europe's elite, pointing out the core of this squad took them to the last 16 last term.
"I have a group of players I want to work with," he said. "I want to make them better and to learn from the experience they've gone through this year and hopefully that'll stand them in good stead for next year."