The Dutch champions were beaten 2-1 by the Bhoys in their last Group H fixture and De Boer believes that to stand any chance of reaching the knockout stages, they must come away with all three points when they meet in Amsterdam.
"How big is the match? Well, if we want to have a meaningful say in this group, we must get three points, that's clear," Sky Sports News quotes him as saying.
"It would take us a point ahead of Celtic. Also, if we presume Barcelona beat AC Milan at home, we can still have a chance of second spot.
"For Celtic, a draw may be enough and they'll go into it more confident than us as we're the team under pressure right now."
Ajax have picked up one point from their three Group H matches this season.