"If you pray, please keep him in your prayers," said a tweet from Duncan's radio show, The Turn, in St. Louis. "If you don't, keep him in your thoughts."
Duncan, 37, was first diagnosed with glioblastoma in 2012, and the cancer returned last year. His mother died of the same illness in 2013 at age 64.
Duncan played for the Cardinals from 2005-09 and was part of the 2006 World Championship team. He became a radio personality soon after he retired in 2010.
His father, Dave Duncan, was a major league catcher and longtime MLB pitching coach.
--Field Level media