Villa will go into Saturday's derby at home to West Brom looking to improve on conceding 11 goals in their last four matches, including Monday's 3-0 reversal at Liverpool.
With Villa only two points above the bottom three, Houllier knows it is imperative his side tighten up defensively.
He said: "At the back we need a bit of both - a change of fortune and of form. We've conceded 11 goals in four games. It's not good enough for a team of our standard.
"We have to get into the games with the idea of keeping a clean sheet because when you keep a clean sheet you give yourself a chance to win. Keeping clean sheets has been an obsession of mine in my career."
Source: PA
Source: PA