The defeat gives rivals Rangers the chance to climb on top of the Ladbrokes Premiership table with a victory against Hamilton later this afternoon.
Lennon bemoaned his side's lack of quality in attack after seeing Christie exit the field midway through the first period.
He told Sky Sports: "We started the game quite well and then with the red card we had to adjust. But we didn't work the goalkeeper anywhere near enough as far as I'm concerned.
"Did we deserve to lose the game? I'm not sure on the balance on play. But we didn't create enough chances to win the game.
"It's a bad day, bad result. You always worry about it off the back of a big European night."
The Celtic boss had no complaints over Willie Collum's decision to send off Christie
He added: "The red card is a major turning point in the game. I have no issue with it. It's late and it's high."