United were hammered in the second half at Old Trafford as City ran riot after the home side lost Jonny Evans to a red card in the 47th minute for a foul on two-goal hero Mario Balotelli as the last defender.
Ferguson was completely devastated after the match, which saw Sergio Aguero, Edin Dzeko (2) and David Silva all score for Roberto Mancini's side, whilst Darren Fletcher got one back for United but it was nowhere near enough.
"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. That's challenge for me too.
"I can't believe the scoreline. The first goal was a blow for sure but it was retrievable at 1-0.
"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," he added.
"It was a bad day."