In exchange, the Tigers obtained right-handed pitcher Billy Buckner.
Willis, 28, has been a bust for Detroit since he was acquired in 2007 along with Miguel Cabrera in an eight-player trade with Florida. In 24 games over three seasons with the Tigers, Willis managed only a 2-8 record and 6.86 earned-run average.
The two-time all-star and 2003 National League Rookie of the Year was signed to a three-year, $29 million contract by Detroit but couldn't find the plate. He was placed on the disabled list twice in 2009 for an anxiety disorder, but Willis denied the diagnosis.
Buckner, meanwhile, has appeared in 36 career games, starting 21 of them, and holds a 6-11 mark and 6.25 ERA in four seasons with the Royals and Diamondbacks.