City are certainly not looking as strong as in the past two seasons, when they had the best defensive record in the league.
Many have pointed the finger at new defensive coach Angelo Gregucci as part of the problem.
But Mancini said: "No, no, no, stop, stop, stop. I took another assistant manager who works with us but in two months he has never worked with the defenders. Maybe for one day. He's not the problem."