Celtic did no significant formation or set-piece work in training because they knew anything worthwhile would be immediately reported back to Milan coach Massimiliano Allegri. Lennon would not reveal whether Anthony Stokes or Teemu Puuki would be preferred as the front man in Celtic's attack although the manager has made up his mind about who will start the opening Group H fixture in northern Italy.
The Scottish champions flew into sunny Milan yesterday afternoon although James Forrest did not travel because of illness and an ongoing sciatica problem.
Joe Ledley was with the group despite a groin problem and Adam Matthews is also in the squad despite suffering an ankle injury against Hearts on Saturday. Nir Biton, the new signing from the Israeli club Ashdod, was due to join his team-mates after the resolution of work permit delays although he is unlikely to feature.
"There are some big decisions to make in terms of the team but I think, in our minds, we already know what it is going to be," said Lennon. "We haven't really had any time to work on it because Monday was still a recovery day from Saturday. Obviously we'll do some light training but we'll not give anything away because I'm sure there will be a lot of people in the stadium looking at it.
"I'm not necessarily trying to keep something up my sleeve but these things do go on when you train at the stadium. When you try to do anything - even like working on set pieces - it is fed back more or less straight away."
Lennon stressed that despite increasing optimism about Celtic's chances - given that Kaka, Ricardo Montolivo and Stephan El Shaaraway are among a number of AC Milan players out injured - he would be thrilled to emerge from the San Siro with a draw.
"A point would be fantastic, especially when you consider that our next game is Barcelona at home. So it's important that we try to get something out of this first game.
"People have said it's not the Milan side of old but I wouldn't read too much into that to be honest. This is a team that beat Barcelona 2-0 in the San Siro last year. At home they're still a pretty formidable team so regardless of a few injuries or pull-outs they'll still be immense opponents to come up against.
"We've got plenty of pace so you're hoping to use that to your advantage. But I want to try to get a foothold in the game and have a fair share of possession. We'll take it from there."
Allegri described his full injury list as reading like "a war bulletin" but did reveal the team to face Lennon's men, albeit with two positions yet to be confirmed.
Allegri has already lost seven regulars to the treatment table, and has yet to make a decision over who will play at full-back and in the wide attacking area. As expected, fromer Manchester City striker Mario Balotelli will the lead the line.
"The formation is almost complete," said Allegri. "We have two doubts: one is between Birsa and Robinho and the other between Constant and Emanuelson."