Willis, who began his career with the Cubs in 2000, had retired last season after a short stay with the Baltimore Orioles.
The Chicago Tribune said the new deal is contingent on Willis passing a physical and does not guarantee he will be invited to the Major League training camp.
Willis enjoyed his best days after being traded to the Marlins in 2002. He won 22 games in 2005 but soon came back to Earth and was 1-6 for Cincinnati two seasons ago.
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