Pep Guardiola admitted he did not see Manchester City's staggering collapse in form coming after conceding twice late on to lose 2-1 to Manchester United on Sunday.
City have won just once in 11 games in all competitions, a run that includes eight defeats.
After an unprecedented four consecutive Premier League titles, City's defence of their crown looks over before Christmas.
Guardiola's men trail leaders Liverpool, who have a game in hand, by nine points and sit outside the Champions League places in fifth.
"I don't have defence, I'm the boss, I'm the manager. I have to find a solution and I don't find a solution," said Guardiola.
"This is a big club, when you lose eight out of 11, something wrong is happening. What can I say? The schedule is tough, the injured players? No.
"I'm the boss, I'm the manager and I'm not good enough. It's as simple as that."
The manner of City's latest loss was one of the most damaging as the English champions were headed for a much-needed victory against their local rivals thanks to Josko Gvardiol's first-half header.
However, they self-imploded as Matheus Nunes' slack pass played in Amad Diallo and the Portuguese midfielder then chopped down the United winger in his attempt to make amends.
Bruno Fernandes converted from the penalty spot to equalise on 88 minutes.
Diallo then raced onto a simple long ball over the top to complete a remarkable turnaround and deepen a spiralling crisis for the club that has dominated English football over the past decade.
"I am sitting here in the press conference because of what we have done in the past," added Guardiola.
"I knew that it would be a tough season. From the beginning, I said many times. even when we were winning.
"But I didn't expect it would be so hard, like it is right now."