Back-to-back games against the Dutch outfit - who sit on one point - will now be crucial to Celtic's chances of repeating last year's feat of reaching the knockout stages of the tournament.
Asked if this was a must-win game for his players, Lennon said: "No, it's a must not get beat.
"I'm confident with this team that we can perform away from home as well, however winning this game would put us in a really good position in the group.
"Losing the game would probably end our aspirations for trying to qualify."
With Adam Matthews out for the rest of the year after shoulder surgery and skipper Scott Brown suspended, Lennon was dealt a further selection blow with the news that Kris Commons is out of this match with a hamstring strain suffered in the 1-1 draw against Hibernian.
The Celtic boss said: "We are hoping it's not as bad as a tear but he won't make tomorrow.
"He might make the corresponding game in Amsterdam, though.
"He's been for a scan and we are waiting on confirmation but he's feeling a lot better today."
The absence of Commons could mean more of an onus on James Forrest to provide the creative spark, after he recently returned to action following a virus.
Lennon said: "He's an important player. What has delighted me is that he looks fit.
"The virus took a lot out of him but when he plays and he's fit and healthy, he's a huge player for Celtic. It's great to have him back.
"The timing is compounded by the loss of Kris but having James back is such a huge boost for everyone."
Lennon, who watched Ajax draw 1-1 with FC Twente in the Eredivisie at the weekend, added: "It's going to be difficult anyway, regardless of personnel and whether we had Scott and Kris.
"I watched Ajax on Saturday night and they were very strong in the second half and probably deserved to get something out of the game.
"They are going to give us problems with the way they move the ball, with the way they play the game.
"They've got good pace in their team in wide areas but we are ready and looking forward to it.
"We know what's at stake but the fact we are at home gives us, I think, a slight heads up."
Ajax will bring a young side to the east end of Glasgow and Lennon hopes Celtic's fans will ensure they are confronted with an intimidating atmosphere at Parkhead.
He said: "It will be a different environment for them and a different atmosphere to what some of them are used to.
"Barcelona are far more experienced and knew what to expect, having been here last year.
"For some of the Ajax players, playing here for the first time, it might be something new.
"We know our fans will bring a huge amount of noise and passion to the stadium and I'm hoping that will fire our players up.
"Not that they need motivated for the game but it will just give them that extra spring in their step."
- Sports & Recreation
- Kris Commons