Genworth Financial may need to minimise the delay in the mooted sale of a 40 per cent stake in its Australian lender's mortgage insurance business if it is to maintain the credit rating of the latter.On Friday, Standard & Poor's revised its outlook on the AA- long term financial strength and issuer credit rating assigned to Genworth Financial Mortgage Insurance Pty Ltd to negative from stable. The rating action followed the downgrading of Genworth Group's core life insurance companies in the US."The outlook revision reflects our opinion that ongoing stress at the group's holding company level has heightened the importance of contributions from the group's subsidiaries, including Genworth Australia," S&P credit analyst Lucy Huynh wrote in an announcement regarding the company's outlook. "We have not lowered the ratings at this time because we expect the company's planned partial sale to take place within the first half of 2013."If the partial sale occurs as planned the rating outlook should revert to stable. If the sale is delayed, or cancelled, a one notch downgrade to A+ is likely.