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
Goals from Gerard Pique and Pedro put the hosts into a 2-0 lead inside 40 mins, before Neymar's hat-trick and a Cristian Tello strike fired Barca well out of sight.
Giorgos Samaras scored an 88th minute consolation for the visitors but it mattered little, as Lennon was left furious with Celtic for falling to their fifth defeat in six Group H matches.
"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."