Ferguson was left shellshocked after City ran riot against 10-man United, finishing 6-1 winners to score their biggest win at the stadium in 85 years.
"It was our worst-ever day," said Ferguson. "It's the worst result in my history, ever. Even as a player I don't think I ever lost 6-1.
"I can't believe the scoreline. The first goal was a blow for sure but it was retrievable at 1-0."
A furious Ferguson, who saw Jonny Evans red-carded, was unhappy that his side did precious little to stop the flow of goals going in the United net.
"The sending off was a killer for us. We kept attacking when we went 4-1 down and we should have just said: 'We've had our day."
"We just kept attacking. They were attacking three versus two. It was crazy football," Ferguson told Sky Sports.
"I thought with the experience we've got - Rio Ferdinand, Patrice Evra - they would (defend more) but we just kept attacking. Sometimes there has to be common sense about it. It was a bad day."