The incredible defeat is the first time since the 1930s that United have conceded six at home and marks their worst home defeat since losing 5-0 to City back in 1955.
The United boss was under no illusions at the impact of the result, admitting it was the worst defeat in his career and in the club's history.
“It was a bad day, our worst ever day,” Sir Alex told Sky Sports. “But we’ll come back from that, that is the challenge.
“The thing that concerned me the most was the goals, for and against. The difference was 10 goals, so that was a big blow for us because you never know if goal difference will count.
“Most years it’s in our favour, but at the moment it’s not.
“We will react. There is no question about that. It’s the perfect result for us to react to because there is a lot of embarrassment in the dressing room today and that will make an impact.
“I feel disappointed. It’s my worst defeat in history, even as a player, I don’t think I’ve ever lost a game 6-1, but that’s a challenge for me too.
“They are all disappointed in there; that will make a difference.
“By January we will be okay, we usually get a shot along the road and then usually do well in the second half of the season and that will have to be the case again.
“At the moment we’ve played all the teams that are around us, so we can’t really make a difference.
“They have all got to play each other obviously, but the second half of the season is usually very important for us.”
Sir Alex’s men went down thanks to two goals each from Mario Balotelli and Edin Dzeko, whilst David Silva and Sergio Aguero also added a goal each, with Jonny Evans receiving a red card early on in the second half.
The Ulsterman received a straight red card for pulling down Balotelli when the Italian got in behind him, but referee Mark Clattenburg had no option but to send the defender off, with Sir Alex believing it played a massive part in the match.
“I think the sending off did play a big part,” rued the 69 year-old. “You can’t believe that scoreline if you see it for the first time.
“For the first 25 minutes we were outstanding and dominated the game. Of course they scored the first goal and that was a blow to us, but the win was still achievable in that situation.
“The sending off was certainly the killer for us and after that, we just kept attacking. In the nature of Manchester United that is all well and good, but once it gets to 4-1 the defenders should just say ‘that is it for today’.
“But we just kept attacking, with full-backs bombing forward and they were effectively three against two – it was crazy, crazy football.
“I thought with the experience we have got in the side, particularly with Rio Ferdinand and Patrice Evra, they would have realised, but they all just kept attacking.
“It is okay playing like the history and tradition of the club, but sometimes you’ve just got to have a bit of common sense.
“It could have been even more for both sides - it was so open, we just kept attacking - unbelievable.”