The German champions host their Portuguese counterparts in Tuesday's quarter-final first leg, with the return match taking place next week.
"They haven't conceded many goals and it is going to be an interesting game," said Guardiola, whose team are leading the Bundesliga by five points.
"I don't feel like we're favourites."
Guardiola, who will become Manchester City coach in the summer, is trying to win the Champions League with Bayern for the first time.
His team were beaten by Portuguese opposition last season at the same stage when they lost 3-1 at Porto before winning 6-1 at home to reach the semi-finals.
Bayern forward Mario Gotze has described the tie with Benfica as "dangerous".
"We have to remind ourselves about what we experienced in Porto," said Gotze.
Bayern will be without Arjen Robben (groin), but fellow winger Kingsley Coman (leg) should be fit.
Benfica, who are top of the Portuguese league, are in the quarter-finals for the first time since they lost 3-1 on aggregate to Chelsea in 2012.
"We go into this game knowing our opponents are candidates to win the Champions League, but we won't be backing down," said Benfica boss Rui Vitoria.
"Any team that make this stage won't say they don't want to win the competition.
"We will think about beating Bayern, regardless of how strong they are."
Bayern Munich will have to break down "perhaps the best defence in Europe" when they meet Benfica in the Champions League, says coach Pep Guardiola.