The 26-year-old Argentina forward injured his thigh in a 2-0 Primera Division win over Almeria on Saturday and has been ruled out for between two and three weeks.
Hoops boss Lennon, whose side lost their Group H opener 2-0 to AC Milan at San Siro, said: "It is obviously very good news for us because he is the best player in the world.
"However, it is tinged with sadness because it is always fantastic to see the great players play here in Glasgow.
"We are looking forward to the game, we welcome Barcelona with open arms and we are hoping it is going to be another great occasion."
Neymar is likely to take over Messi's superstar mantle for the evening and become the focus of the Barca attack but Lennon does not believe the two players can be compared.
The former Parkhead skipper said: "It is like comparing a dog with a cat.
"They are two different types, two different players altogether.
"He (Neymar) is wonderfully gifted, quick, predominantly playing on the left-hand side and he has goals in him.
"He is only young and he has a fantastic future but it is far too early to make comparisons to Messi.
"There is only one player currently who could arguably be around the calibre of Messi and that is Cristiano Ronaldo."
Lennon, whose side famously beat the Catalan giants 2-1 at Celtic Park last November on their way to the last 16 of the competition, believes Barca will be instantly recognisable even without their Argentinian superstar.
"Our approach will be the same whether Messi plays or not," said the Northern Irishman.
"Barcelona don't change their system, they don't change their style of play really and 4-3-3 is just what they believe in. It has worked well for them very well over the years.
"It would be fantastic if we could get something out of it like we did last year.
"We didn't really budget for it, as it were, in both the games last year, so if we can get something it will be a huge bonus for us.
"It will be very much at times containment; to limit Barcelona to as few chances as we can and at the same time maximise the opportunities that we get, as we did last year.
"But they are not like any other team. Against Milan, we had our fair share of the ball and had our chances.
"But Barcelona, for me, are a class above in the way they pass the ball, the way they create chances and the pace they have in wider areas, and that is not being disrespectful to Milan.
"I think they have been the best club side in the world for possibly the last 10 years although Bayern Munich look to have taken over that mantle."
Aside from missing Messi, the visitors head into the tie likely to be without defenders Javier Mascherano, Jordi Alba and Carles Puyol.
However, Lennon shrugged off the assertion that Barcelona will be weaker at the back.
He said: "You say that they have defensive problems with injuries but they keep on keeping clean sheets; against Rayo Vallecano, Real Sociedad and Almeria. So, regardless of personnel, they are still defending very well.
"They are unbeaten domestically, they got off to a great start in the group (4-0 win over Ajax). They are a fantastic team.
"They will be one of the favourites to win the trophy. Whether they are better than last year, you won't be able to gauge that until the turn of the year."