Kelly's wife, Jill, posted an update online Wednesday saying the treatment plan for her husband has changed because the cancer is in areas from which it cannot be eradicated by surgery.
She says doctors believe chemotherapy and radiation are the best option given the cancer's complexity and aggressive nature.
The 54-year-old Buffalo Bills star underwent surgery last June to remove a squamous cell carcinoma from his upper jaw. He had been expected to undergo additional surgery this week or next week at a New York City hospital.
Kelly spent 11 seasons with the Bills and was inducted into the Pro Football Hall of Fame in 2002.