The Hammers, who are currently one point adrift of safety in the table, make the trip to Stamford Bridge to face their London rivals.
Despite being underdogs for the game, Demel believes that the visitors have what it takes to get a positive result against the Blues.
"Every game is winnable," he told the club's official website. "It depends how prepared you are and how focused you are.
"I think that even Chelsea is a winnable game. Of course they're a big team but it's a football game and when I step onto the pitch each weekend I'm not expecting to lose - no matter which team is in front of me.
"That's not the way I grew up and it's not the way I see things."
Demel could return to the West Ham squad for the match after suffering concussion against Cardiff City at the start of the month.