Richards is back in the England squad this week.
"When Mancini took over it was a hard place to be because with all the money the owners brought in you didn't know whether you were going to be part of his plans," he told the Guardian.
"He's improved my game a lot. He always tells me: 'If you're 100% you can be the best.' When a manager's telling you that, you believe him. He had all the money to spend and could have gone out and bought a right-back but he has me and Pablo Zabaleta - we share it out. He's stuck with me."