The Bees earned a credible draw at Griffin Park on Saturday, but Donaldson says that the tie is far from over.
The forward says that his side will go to Stamford Bridge and hopefully keep the crowd quiet.
"Their run of form at home recently hasn't been the best," Donaldson told the Daily Star.
"The pressure will be on them again and hopefully we can go there and do the job that we did at home.
"Obviously there is unrest from the fans with the manager. Hopefully we can put that to our advantage and spark some unrest among their fans."
The replay will take place on February 17.