Cesare Prandelli's men can take a huge step towards automatic qualification from Group B for Brazil 2014 with a win at Stadio Renzo Barbera in Palermo.
Buffon and his side sit four points clear of Bulgaria with four matches to play but the 35-year-old Juventus goalkeeper is concerned about his team's chances against Lyuboslav Penev's men.
"I'm quite worried for the match against Bulgaria, as always before matches that become decisive for a qualification," Buffon said on Wednesday.
"We deserve the qualification for what we've done up to now but we haven't gained it yet. We will have to prepare and face the match with great attention to take another important step forward."
In PSV's Stanislav Manolev and Aston Villa signing Aleksandar Tonev, Bulgaria have two of the group's top scorers with three goals each.
Buffon is wary of the individuals in Bulgaria's side and warned his team-mates the visitors would try and punish them on the counter-attack.
"Bulgaria are a team with two or three important single players but they're also a very close group that knows very well how to face these kinds of games, trying to defend well and counter-attack," the veteran goalkeeper said.
"They have three or four good runners and they're in good condition. We need to be good at limiting them."
Buffon was full of praise for Prandelli, who may step down after the 2014 World Cup according to reports in Italy.
"Prandelli's touch was and is very important because he made us believe that we can try and reach aims and win also having fun, playing well and improving the technical aspects of a game," Buffon said.
"It's been a turning point compared to our past."
