The Scottish champions claimed a 2-1 win when the teams met at Parkhead two weeks ago, but Lennon has admitted that his side are in for a tough night.
"We always knew that the double-header against Ajax would be pivotal and it has proved to be the case," Sky Sports News quotes him as saying.
"You don't know what is round the corner but what I would say is that on their day Ajax are as good as Barcelona and Milan.
"They caused us as many problems as both those teams and I think it will be the same on Wednesday night."
Celtic are currently third in their Champions League group, with three points.