Melbourne's All-Australian key defender James Frawley is set to miss the start of the AFL season after sustaining a serious pectoral injury at training.

Frawley, 22, tore the left pectoralis major tendon on Thursday and will undergo surgery next week.

He is expected to be sidelined for 10-12 weeks.

Melbourne's opening match of the home and away season is against Sydney at the MCG on March 27.

"This is obviously an unfortunate setback for James," said Melbourne coach Dean Bailey.

"He is clearly an integral part of our team and he will be missed.

"However, we have full confidence that others around James will step up to fill the void in his absence."

Frawley had the best season of his AFL career in 2010, winning All-Australian selection and finishing second to Brad Green in the Demons' best and fairest award.

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