The four-time defending Premier League champions have lost eight of their last 11 games in all competitions – winning just once in that run.
Having played a game more, City are 9 points behind Liverpool and in fifth place in the table.
“I’m the boss, I’m the manager and I’m not good enough. It’s as simple as that,” Guardiola said after the latest defeat. “I’m not doing well. That’s the truth.”
Guardiola has led City to unprecedented success, winning four Premier League titles in a row and six in the last seven seasons. He has won 15 major trophies at the club, including the Champions League.
But he is experiencing the worst form of 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 it has failed to boost his team’s form.
City went 1-0 up against United in the 88th minute, but late goals from Bruno Fernandes and Amad Diallo – the fans cheered after the final whistle.
“I knew it would be a difficult season from the start. I said (that) many times, even when we won, but I didn’t think it would be as difficult as it is now,” Guardiola said.