Top St Kilda defender Sam Fisher will miss up to a month of the AFL season with his ankle injury.

The 2008 All-Australian had an operation on Monday after hurting the ankle in Saturday night's NAB Cup final loss to the Western Bulldogs at Etihad Stadium.

"St Kilda can confirm that Sam Fisher this morning had an arthroscope on his right ankle to assess the injury sustained during Saturday night's final," the Saints said in a statement.

"His estimated recovery time is four-six weeks."

That will put him out of action possibly until round five.

Saints coach Ross Lyon revealed during Sunday's club family day function that the injury was worse than they originally thought.

They are hopeful that vice-captain Lenny Hayes will be fit for round one after missing the Cup final.

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