Gibson's agent, Dick Zimmerman, told the Post-Dispatch he expects Gibson to begin chemotherapy Monday in his hometown of Omaha, Neb.
"We all know what a competitor he is," Zimmerman said.
Gibson was 251-174 with a 2.91 ERA and 3,117 strikeouts from 1959 to 1975. He played his entire career with the Cardinals and won two World Series titles in St. Louis.
Gibson had a 1.12 ERA across 304 2 /3 innings in the 1968 season and led the league with 268 strikeouts that year. He was a nine-time All-Star, a nine-time Gold Glove winner and threw a no-hitter in 1971.
Gibson was inducted into the National Baseball Hall of Fame in 1981.
The Cardinals tweeted their support Saturday night, writing, "All of Cardinal Nation is behind you!"
The stage of Gibson's cancer wasn't revealed.
The five-year survival rate of pancreatic cancer is 9.3 percent, according to the National Cancer Institute. If it is confined to the primary site the rate is 37.4 percent but it's 2.9 percent if the cancer has metastasized.
In March, Jeopardy! host Alex Trebek announced he had stage 4 pancreatic cancer. Two months later he said some of his tumors have shrunk by 50 percent after undergoing chemotherapy. Doctors say he is near remission.