The Hoops boss admitted that the Spaniards were "fantastic", but described his own team's display as "half-hearted".
"Barcelona were fantastic as we expected them to be but we were very poor. There was no bravery on the ball and it looked like our heads went down at 2-0," BBC Sport quotes him as saying.
"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."
Georgios Samaras hit a late consolation for Celtic, who were already eliminated for the competition ahead of kickoff.