The Parkhead side will be without their captain for Wednesday's game against Ajax in Amsterdam and will still have to contend with his absence for the penultimate game of the group against AC Milan at Celtic Park. He will return for the final clash with Barcelona in Spain.
Brown was handed the suspension for taking a swipe at Neymar when Celtic hosted Barca last month. But, given the severity of the penalty, and the fact that Brown himself appeared in person in front of the panel, Celtic had hoped for some level of commonsense to prevail.
Asked if Brown being suspended could be the difference between qualifying and not, Lennon said: "Absolutely, he is a huge player for us and we are going to miss him. It's another two games now and Ajax away is pivotal.
"Look, we are prepared for it. But it's still very disappointing because I felt we had a good case. I'm sure we had a fair hearing but what can we do?
"It's a shame because he has been playing out of this world. For him to miss these games is a blow to him more than anybody - and a blow to us."
As Lennon prepares for the game in Amsterdam without Brown, he has stressed just how pivotal an encounter it is for Celtic.
Avoiding defeat keeps their ambitions of further European involvement after Christmas going, while a win would restore their ambitions of getting out of the group.
"If we can get something in Holland we give ourselves a great chance for the last two games," Lennon said.
"But if you get beaten by Ajax then qualifiying might be beyond us - depending on other results. If we come back with a point or, even better, three, it sets Milan up to be something special."