The League One side will host the Europeans Champions at Griffin Park in the first meeting between the two sides in 63 years.
Brentford's manager told Sky Sports News: "They will take the game seriously. They can only win the FA Cup and the Europa League and a club like Chelsea has to win trophies each year.
"Chelsea will still have an extremely strong team. I still think their players will underestimate us. It's a human reaction. I was as a player the same and I think that is our chance.
"It's a game that you will play against a team that is close and it will be a massive, massive motivation for them too."
Last time the sides met, Chelsea won 1-0 in the third round of the Cup.