"It's not good enough for a team of our standard," admitted Houllier.
"We have to get into the games with the idea of keeping a clean sheet because when you keep clean sheets you give yourself a chance to win."
Villa's latest setback came at Houllier's former club Liverpool on Monday, when they suffered a 3-0 defeat which left them a lowly 16th in the table.
The Frenchman said: "At the back we need a bit of both - a change of fortune and of form.
"I won't tell you what I put our defensive problems down to. I think it is collective really. We need to be better.
Why does our defence seem to be getting worse every game?
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"We obviously need to eradicate some stupid, schoolboy errors at the back which cost goals.
"We are improving in terms of our passing, movement and preparation, although there are some areas where we have to keep improving.
"We need more of a cutting edge and to be more clinical at times."
Albion, promoted to the top flight this season, are five points clear of Villa after back-to-back wins over Everton and Newcastle.
But manager Roberto Di Matteo insists league positions mean little at this point in the campaign.
"I don't think it [being above Villa] counts for very much at this stage of the season," said the Italian.
"We are delighted with the points we have [22] by the middle of December. We are just going to try and add some more as soon as we can. That's our focus."
Source: BBC Sport
Source: BBC Sport