"Every keeper is like this," said Mancini. "It is important because it's different to other positions. If you want to be a keeper, you should be cocky. Joe is a good guy, he works very hard, he wants to improve.
"Last week [Wayne] Rooney said on Twitter that Joe is the best keeper in the world but today he is called cocky.
"It is my opinion that Joe is the best keeper in England and one of the best in Europe. Sometimes a keeper can make a mistake but this is normal. If you want to be a good keeper, you should be cocky sometimes."
City are at West Bromwich Albion in the Premier League on Saturday.