The four-time defending Premier League champions have lost eight of their last 11 games in all competitions - winning just once in that run.
City are fifth in the standings and nine points behind leaders Liverpool, having played a game more.
"I'm the boss, I'm the manager and I'm not good enough. It's as simple as that," Guardiola said after Sunday's loss.
"I'm not doing well. That is the truth.
"This is a big club and when you lose eight of (11) something is wrong. You can say the schedule is tough or the injuries to players, but no."
Pep Guardiola consoles Matheus Nunes after City's late collapse against Manchester rivals United. (AP PHOTO)
Guardiola has led City to unprecedented success, winning an unprecedented six Premier League titles in the last seven seasons. He has won 15 major trophies at the club, including the Champions League.
But he is experiencing the worst run of form in his coaching career, which has seen him win 33 major trophies at Barcelona, Bayern Munich and City.
Guardiola signed a two-year contract extension last month, but that has not sparked an upturn in his team's form.
City led 1-0 going into the 88th minute against United, but then conceded late goals to Bruno Fernandes and Amad Diallo — prompting boos from fans after the final whistle at Etihad Stadium.
"I knew that it would be a tough season from the beginning," Guardiola added.
"I said (that) many times, even when we were winning, but I didn't expect it was so hard like it was right now."
33°C