The outgoing Pellegrini says his single season at Real Madrid was the “most painful” period of his coaching career.
City host Real offering the Chilean a chance of revenge against the team he coached in 2009/10.
He says: “My time at Madrid was the most painful. The team was not complete, it was still in a phase of formation and had some evident problems.”
Madrid spent £250million on players in the summer of 2009 when Pellegrini took over, including Cristiano Ronaldo, Kaka, Karim Benzema and Xabi Alonso.
However, Pellegrini criticised the sales of winger Arjen Robben to Bayern Munich and midfielder Wesley Sneijder to Inter Milan and also said he had no relationship with the Madrid board and president Florentino Perez.
“I knew they were going to sack me since I arrived, since August. I had no relationship with the board.”
CR7, meanwhile, faces a race against time to shake off a thigh injury to be in Zinedine Zidane’s team at Eastlands.
A club statement says: “Ronaldo has been diagnosed with a muscle strain in his right thigh. His recovery will continue to be assessed.”