His condition was discovered on Wednesday after a visit to the hospital because of an infected thumb. The possibility of Hodgkin's disease was considered and further tests confirmed it.
According to the medical information, Fields' prognosis for recovery is excellent.
"I'm truly blessed to have found out in this stage," Fields said. "Right now, my main goal is to focus on my condition. I don't look at this as a setback, but another of life's ultimate challenges."
Fields, 30, was signed as a free agent by Carolina before the 2002 season. He was the Panthers' starting strongside linebacker last season, and led the NFL's second-ranked defense last season with 127 tackles and registered 7 1/2 sacks.
Fields spent his first six years with New Orleans and was selected to the Pro Bowl in 2000. He signed with St. Louis as a free agent in 2001.
Also, defensive tackle Brentson Buckner has undergone arthroscopic surgery on his right knee and is expected to miss the team's final two preseason games.
Notable deaths of 2014 [PHOTOS]