Bull has kept clean sheets in his last two games but Winfield said: "You can't expect your goalkeeper to get you out of trouble every time.
"You have to stop the shots and minimise their chances as much as you can.
"I am disappointed, personally, because they keep having more shots on goal than I would like to see. To keep clean sheets is our aim as a defence so we have to take the positives.
"But in all of our games there have been chances where the other teams could have scored and it is not good enough and we need to work on the training ground to sort it out before it costs us."
Wycombe's last trip to Sixfields was in January 2006 when the two sides fought out a goalless draw in League Two.