The Chilean tactician, currently in charge of Hebei China Fortune, admits he regrets announcing his departure too early.
Pellegrini told the Coaches Voice: "I did make one mistake in my time at City.
“As much as you don't want it to be this way, the group has a tendency to relax. To start thinking about the next step.
"I said I would end my contract after three years. After that, I said we would see.
"There is a lot of trust between those two and myself, so I was always kept in the loop about the plans to try and bring Guardiola to the club.
“I knew that if he hadn't wanted to come to City for any reason, they weren't going to interview for a manager. They would come straight to me.
"But I learned something from the experience in my final season. It's the same in the case of Sir Alex Ferguson.
“With all the experience and influence he had at Manchester United, he made his first retirement announcement several months early.
“I think he always described it as a mistake in his career. I also describe mine as a mistake."