Strategist Sir Alex Ferguson admitted that Sunday's loss to the club is the "most painful in his career."
The English champions were crushed by even 6-1 in front of their own fans. The hosts of the "Old Trafford" stadium had not suffered such a painful blow since 1955.
"This is the worst day," said Ferguson. "I have never had a worse result in my career. I can't remember such a defeat even from the times when I was a player myself. So, this will be a big challenge for me.
"I can't believe we lost like this. The first goal was painful, but we had the opportunity to bounce back. We were shattered by the red card."
Defender Jonny Evans was sent off the field at the beginning of the second half, and "Man City" was led to victory by two goals from Mario Balotelli and Edin Dzeko, as well as goals from David Silva and Sergio Aguero.
"We were attacking even when the opponents were leading 4-1, although we shouldn't have done that. That's why 'Man City' prepared fast attacks, with three attackers breaking through against two defenders," the specialist noted.
And he continued: "I think we have enough experienced players, such as Rio Ferdinand or Patrice Evra, so they had to understand to defend more, but they still wanted to attack. Sometimes you need to use common sense."