Barton will miss the Manchester derby

Keegan told the City website: "It is a blow for everybody and he will be out for at least six weeks.

"It's very similar to what Sun Jihai has had. When the specialist looked at it he said Joey would have to have an operation on his cartilage."

Barton is unlikely to be unavailable until early December.

He will miss the Carling Cup tie against Arsenal and the first Manchester derby of the season at Old Trafford next month.