The Hammers, who welcome the Blues to Upton Park on Saturday, caused an upset in their last match as they beat Tottenham Hotspur 3-0 at White Hart Lane.
However, Jaaskelainen admits that his teammates will have to be at the top of their game if they are to spring a surprise for the second consecutive fixture.
"If City turn up and play their best it is going to be extremely difficult for us to do anything, but we will see," he told the club's official website.
"I wouldn't take a draw now, but if things don't go our way and City plays a good game, it is going to be hard to beat them, but we have to make sure that we are hard to beat as well."
West Ham are currently in 13th place in the Premier League table.