The Championship strugglers have conceded 45 goals in just 31 matches so far this season.
As a result, Saunders has conceded that he will be risking his job unless he makes improvements in that area of the pitch.
"This team has been conceding too many goals for three years. I have to change it as quickly as I can," he told BBC West Midlands. "Three managers been sacked here because the team can't keep a clean sheet.
"I recognised what the problem was, I've been trying for three weeks and I've got to get some players in that we can afford."
Saunders's team are just one place above the relegation zone.