Willis, who sang songs such as "YMCA" and "Macho Man" with the costumed pop group until 1980, had an operation in San Diego last week to remove nodules from his vocal cords.
"I feel great and look forward to performing before the end of the year but I can't say for sure," the 57-year-old singer said in a statement.
His spokeswoman told the BBC that Willis' doctors instructed him not to sing for 90 days, even though his speaking voice "sounds almost normal."
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