The former Buffalo Bills star made the announcement at his football camp in Buffalo on Monday and his comments were posted on the Bills' website.
Kelly says he found out the news on Wednesday.
"From what I've been told, everything that they did in surgery, if I had to do radiation or chemo it would put me way back," Kelly said.
He said he had the left side of his jaw and the teeth on that side of his mouth removed in surgery on June 7. Kelly was released from the hospital three days later.
"It's very, very sore, but it takes time," he said. "I know when you have knee surgery or shoulder surgery it's sore for a small period of time, and rehab and everything is good and you just take your time. This is just constant pain for now, but it's a small price to pay for where I'll be later on down the road."
He says he's scheduled for a follow-up with doctors in two months to see if the cancer stays away.
"I never thought I'd be saying cancer. I never thought the `C' word would become part of my vocabulary," Kelly said. "But it is what has happened to me and the ups and downs of my life. I'm already on the way back up."
The American Cancer Society estimates about 2.2 million Americans are diagnosed with this form of cancer each year. The society adds that death from these types of cancers is uncommon. About 2,000 people die each year, and that rate is dropping.
Risk factors include smoking and alcohol consumption.
Kelly's diagnosis stems from pain he began experiencing in his jaw in December. He initially thought it was an infection, but grew concerned when antibiotics failed to help.
Tests eventually led to doctors removing a nickel-sized cyst from his gums and nasal cavity during an operation in early March. Follow-up tests revealed the cancer.
It's the latest operation Kelly has required over the past two years. He's also had surgery to correct back, neck and hernia problems.
Inducted into the Pro Football Hall of Fame in 2002, Kelly still holds nearly every significant career Buffalo Bills passing record: 35,467 yards, 237 touchdowns and 26 300-yard games.
Kelly spent 11 seasons with the Bills before retiring following the 1996 season, and has since made Buffalo his home. Known for his fearless, swashbuckling style, Kelly was the face of Bills teams that made four consecutive Super Bowl appearances in the early 1990s, only to lose them all.
Kelly intended to draw upon his faith and family, and the perseverance he's developed in facing other challenges in his life both on and off the field. Kelly's son, Hunter, was born with Krabbe disease, an inherited degenerative disorder of the central and peripheral nervous systems. Given little more than three years to live, Hunter died at the age of 8 in 2005.
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