The blue half of Manchester have two games in hand on their title rivals, while the Baggies reside three points off the relegation zone with five matches left to play.
Foster believes that the Baggies will receive a huge boost should midfielder Yaya Toure miss the match, as he continues to struggle with injury.
"We will go into the City game genuinely believing we can get something," Foster told the club's official website. "You've got to.
"Yaya Toure came limping off [against Liverpool] so hopefully for us he won't play a part because he's huge for them. But they've got players who can fill in as well.
"We know it's going to be tough, but as long as we keep playing and attacking in the way we have done recently I genuinely believe we can get something. We've got to believe that."
West Brom, who are unbeaten in their last three games, have acquired eight points from five fixtures.