Griffin Park could hold its largest crowd in 30 years if it is full to its 12,000 capacity on Sunday, with few anticipating an upset as Chelsea seek to defend the FA Cup in a season where opportunities for silverware are dwindling.
"They will take the game seriously," Rosler said.
"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."