A 2-0 triumph on the night at Celtic Park gave the Bhoys a 4-0 aggregate victory over their Swedish opponents.
"It's the best thing I've done in football," Lennon told Sky Sports News. "The pressure going into this game and, in particular the last five or six weeks, has been enormous.
"I actually feel British clubs going into these games are at an unfair disadvantage because they're starting their season, so it makes the effort of the players and the performances even more remarkable in the circumstances."
Celtic will now be in the main draw for the competition which takes place on Thursday.