The Hoops earned their place among Europe's elite on Wednesday night by overturning a 2-0 first-leg deficit to knock Kazakhstan champions Shakhter Karagandy out of the play-offs, the 3-0 win at Parkhead made all the more dramatic with James Forrest's tie-clinching goal in injury-time.
The Scottish champions beat Barca last season on their way to the last-16 of the tournament and will return to the Nou Camp as well as two other great European stadiums in the San Siro and the Amsterdam Arena.
After watching the draw take place with his players at the club's Lennoxtown training complex, the Hoops boss said: "It is obviously very exciting.
"I think it's the best and worst group we could have got.
"In terms of glamour, I don't think you could ask for anything more with Barcelona, Milan and Ajax.
"In terms of difficulty, I don't think you could have asked for anything more either in terms of the quality of teams we are going to face.
"But for me the hard work was done last night and the last six or seven weeks and now we can enjoy it - the players can savour it and so can the supporters.
"We are going to be playing three wonderful football clubs.
"Ajax were with Dortmund and Man City (last season) and came third in the group - the group with all the champions in it.
"Milan have been in transition but I think they are coming on strong again. They beat a very good PSV (Eindhoven) team to qualify so their pedigree is unquestionable in this tournament. And we are going to see our old friends Barcelona again. "
The former Celtic skipper tried to ensure expectations of a similar run this season would not be put upon his players.
"I said at the beginning of the tournament it would be very difficult to emulate what we did last year," said Lennon.
"I think qualifying for the group stages was huge for us. If we can get anything else out of it, then so be it.
"You just never know with this bunch of players what they are going to bring - we could get no points we could 10, you just never know.
"The pressure is off in terms of I don't think anyone really has huge expectations on us to qualify, although some of our fans who set very high standards will be expecting that.
"The experience of last year will stand the players in good stead.
"The new boys coming in are going to get a taste of some special occasions. We had a great night last night and have three more at least to look forward to.
"In terms of the football, we are going to come up against three wonderful footballing teams and putting our wits against them is what we all want to do."