The striker, 25, injured the joint during a Hammers training session on their pre-season tour of New Zealand and will undergo surgery on Friday to correct the problem.
Carroll was withdrawn from the squad to face Wellington Pheonix on Wednesday night, which they lost 2-1, after complaining of a sore left ankle and scans have revealed the extent of the damage – which is significant.
West Ham previously released a statement saying Carroll would miss their remaining fixtures in New Zealand as a precautionary measure, but reports suggest he could now be out until November.
"Going under the knife tomorrow morning and absolutely devastated to be missing the start of the season," Carroll tweeted.
Carroll moved to West Ham on loan from Liverpool initially, scoring seven goals in 24 appearances in the 2012-13 season before moving to Upton Park permanently that summer.
But he was plagued by injuries last term, missing the majority with a heel problem and playing just 16 times in all.
West Ham’s second and final game of the ongoing tour comes against Sydney FC in Wellington on Saturday.