The Hoops trailed 2-0 from the first leg of the play-off in Astana but goals from Kris Commons, Georgios Samaras and James Forrest secured a 3-0 victory and a place in Thursday's draw.
Lennon, who revealed he had agreed a fee with Schalke 04 for striker Teemu Pukki, said: "It is the greatest thing I have ever done in football. We lost three very important players (Gary Hooper, Victor Wanyama and Kelvin Wilson) and it has been difficult to replace them.
"Coming from a two-goal deficit as well it makes the players' performances all the more remarkable. The players were wonderful."
The former Celtic skipper then turned his sights on the club's detractors, saying: "I am relieved. There is a great sense of pride in what we achieved considering we lost Hooper, Kelvin and Wanyama, who were the spine of the team for a year and half.
"I am angry as well. I am angry at the reaction from the first leg towards the team. The environment here is all wrong. There is far too much negativity.
"These players have given everything for the club and have put Scottish football in a really good light last year. I wouldn't say we had a bad week last week, we didn't take our chances, we created plenty but the hysteria afterwards was disgusting.
"It might have come from some quarters of the media or some quarters of our own support but the knee-jerk reaction to an average performance was baffling to me. It just makes the job impossible at times.
"There is no respect for managers these days, we are analysed and scrutinised. I call the game as I see it but you still get slaughtered and people try to humiliate you.
"It is totally wrong and it doesn't do the prestige of the job justice."