I'm not sure if this is sarcasm. Apologies if i've been wooshed but 12 clean sheets in a row doesn't resemble 'really average' in my eyes.
Van Der Sar has barely been called on. Did you watch the Everton match. The other team barely attacked, and when they did it was poor. I've watched Van Der sar plenty this year, I can't remember him doing one exceptional thing. He's looked shaky on several occasions too.
Regardless of how good the defence are, they still need a class keeper. I bet you any money if Ben Foster had played these last 12 games they'd have conceded goals.