The Scottish champions face their Dutch counterparts on Wednesday knowing that a win will keep alive their hopes of progressing from Group H.
Celtic earned a 2-1 victory over Ajax when the two teams met at Parkhead two weeks ago.
"The game in Amsterdam is pivotal," BBC Sport quotes Lennon as saying.
"It's a huge game and we started working on it - the team selection and the way we'll play - as soon as Saturday's game [against Dundee United] finished."
Lennon's side made it through to the last 16 of last season's competition before they were beaten by Juventus.