European champions Chelsea will be taking on the League One Bees for the first time in 63 years - Chelsea claimed a 1-0 FA Cup third-round win in front of 38,000 fans in that last meeting - in the west London derby.
"They will take the game seriously," said Rosler.
"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. We have to believe in our chance and we have to go for it."
However Chelsea manager Rafa Benitez appears fully aware of the threat that Rosler's team are likely to pose as he looks to avoid exiting another cup competition after the Capital One Cup semi-final defeat to Swansea.
The Spaniard said: "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.
"I have seen some games and I was talking with the scout. They can play in different ways. They can switch.
"They can play long, they can try to win second balls. So, quite an interesting team and they are doing well.
"But as a top side, you have to win. And you play against a team in another division, so you have to win."