The Scottish champions lost 4-3 on the night – and 5-4 on aggregate – despite leading twice against the Romanians.
Celtic have now exited the competition at this stage in successive seasons, after losing to AEK Athens last term.
“We had the lead and we’ve let it slip through our own decision making,” Lennon told BBC Scotland.
“The goals we conceded are crazy. You’ve got to do the basics well – you’ve got to stop crosses, close people down, get your head on things – and it was almost as if we scored and wanted to switch off.
“We’ve only ourselves to blame. If you don’t do the basics defensively well enough then at this level you get punished. We’ve let it go.”
“We didn’t compete in the first 45 minutes at all,” said Lennon. “The second half we really had a lot more urgency but the goals we conceded are crazy. We needed that two-goal cushion really.
“What surprised me was our approach to the game. We’ve worked hard to get where we are and we’ve basically thrown it away.
“Now we need to look at the Europa League, make sure we qualify first then take it from there.”
Lennon also defended his decision to start midfielder Callum McGregor at left-back in place of summer signing Boli Bolingoli.
“People can point at that but he [McGregor] did set up our equaliser and had a very good game,” the manager said. “I don’t think the left side was the problem tonight.”