The Hoops took on Barcelona at the Nou Camp on Wednesday night, but a lacklustre performance left Lennon facing a 6-1 score which included a hat-trick fired past his team by Brazil star Neymar.
It left Celtic bottom of Group H with just three points from six games.
"Barcelona were fantastic, as we expected them to be, but we were very poor," Lennon said.
"I wouldn't say some players gave up, but it certainly looked that way at times.
"I would never accuse a player of doing that, but a lot of half-hearted performances and as a manager, that was the most disappointing aspect of the whole game."
It was unacceptable from some of the players - nowhere near the performances against Barcelona recently so that was very disappointing, obviously.
Lennon added: "There was no bravery on the ball tonight, we started off nervously and it looked like our heads went down at 2-0.
"We were weak and that has not been like us - that is something I have to address going forward."
It took just seven minutes for Barca to open their account, courtesy of defender Gerard Pique.
Pedro Rodriguez and Neymar added two more goals before the break, with Neymar's second coming just two minutes after the interval.
Substitute Cristian Tello added the sixth just after 70 minutes gone.
It fell to Georgios Samaras to grab a late goal that was little consolation for Celtic.
Barca boss Gerardo Martino was understandably delighted with the result.
"Our play is improving all the time and I expect better by the end of the year. There is always room for improvement and that is what we are striving for," he said.
"Hopefully this is the breakthrough moment this season."