The reaction on Monday really depends on whom knew what last week as far as the downgrade is concerned.
Timeline:
Official report came out when AH session was over so the market reaction can't be known for sure before Sunday session
But the rumors were rampant last week:
"The S&P 500 climbed as much as 1.5 percent in the first five minutes of trading yesterday
"(they mean Friday Open)" before turning lower as
speculation swirled through the market that S&P was preparing to strip the U.S. of its AAA rating for the first time."
"After being alerted to that, S&P lowered its calculations by $2 trillion and then relied largely on judgments about the U.S. politics to support the downgrade, the person said. The Treasury was presented with the S&P analysis shortly before 2 p.m. yesterday
"(Thursday 2pm Eastern?)"."
Looks like S&P decided on the downgrade some time during the day on Thursday Aug 4th.
Basically, the info could have leaked out on Thursday when the market dropped 5%. I believe there was also news about Italy that did not help. But it is quite possible that somebody knew something about S&P downgrade and was dumping on the unsuspecting traders.
The market does not just drop 5% on a rumor. It appears that the rumor was quite credible (and it indeed was) to cause the massive stock market losses.
The bottom line is that much of S&P news has been priced in by Friday close. In fact there were only a small drop in SPY (-0.5%) during AH when CNBC was openly reporting "the rumor".