A win, or even a scoring draw, against Benfica would have sent Celtic into the last 16 of the UEFA Champions League but they now face a tense matchday six when they host Spartak Moscow, needing to better Benfica's result at Barcelona.
Ola John gave Benfica an early lead but that was cancelled out by Georgios Samaras, only for Ezequiel Garay to hit the hosts' winner in the 71st minute.
"I'm disappointed," said Lennon. "I'm disappointed with the two goals we conceded, particularly the second one where we have cleared it but haven't picked up as we were coming out.
"There was a lack of composure about us at times, a nervousness on the ball. But we got a goal and we had chances towards the end to get something out of the game.
"But we have one more crack at it. We know what we have to do and that's to beat Spartak Moscow at home and we will still have a good chance of qualifying.
"They are a young team. This is a hard place to come and play football especially under the circumstances.
"I thought we got a little bit of a lift when we got the goal but second half we came out and never really kept the ball as well as we would have liked. So we put the extra man in midfield and still didn't show enough composure at times.
"I didn't feel a goal was coming, but the amount of time you give the ball away in bad areas, or give it away cheaply, then the pressure is going to come.
"Anyway they got the goal and we decided to throw young Tony (Watt) on and he gave us a little bit of a lift and we almost nicked a draw at the end. But we live to fight another day."
Lennon admitted the loss of Victor Wanyama for the crucial game against Spartak through suspension will be keenly felt.
"It will be a huge loss because he has been magnificent, he will be a big miss," he said.
"I thought some decisions really went against us sometimes but hopefully we will be getting some players back for Spartak. A few are running on empty."