The Scottish side bowed out of the Champions League with a whimper following a 6-1 loss at Camp Nou, blasting his squad's lack of bravery and mental strength
Lennon was furious as Celtic not only suffered a fifth defeat from six Group H matches, but also embarrassment at Camp Nou.
The Scottish champions – who reached the last 16 last term before being bundled out by Juventus – were six points off reaching the knockout stage in this year's competition and finished their campaign on a dramatic low.
"Barcelona were fantastic as we expected them to be but we were very poor," Lennon told reporters.
"There was no bravery on the ball and it looked like our heads went down at 2-0. We were weak and that is something I have to address.
"There were only three players who really showed anything - Mikael Lustig, Virgil Van Dijk and Adam Matthews. I'd excuse the goalkeeper [Fraser Forster] as well.
"There were a lot of half-hearted performances out there and that was the most disappointing aspect.
"I'm angry and disappointed but it's something we need to get over quickly. We are better than that."