Willis, nicknamed the "D-Train," was 72-69 with a 4.17 ERA in nine seasons with the Marlins, Tigers, Diamondbacks and Reds. He won the 2003 National League Rookie of the Year Award and finished second in the 2005 Cy Young Award balloting after going 22-10 with a 2.63 ERA.
"Dontrelle has had a great deal of success at the major-league level," Ducks president and general manager Michael Pfaff said in a news release. "We are eager to add him to our pitching staff and excited to have him join the Atlantic League."
A strikeout pitcher who often had control problems, Willis had issues with walks that reached new heights in 2008 after an offseason trade from the Marlins to the Tigers. He walked 35 hitters in 24 innings that season, beginning his fall from the majors. He later was diagnosed with anxiety disorder.
Willis pitched at least 197 innings from 2004-07 but never threw more than 752/3 in a season after that. He was 1-6 with a 5.00 ERA in 13 games (all starts) for the Reds in 2011, his last MLB season, and went to spring training with the Cubs before this season began. But that comeback attempt lasted only seven pitches before Willis was removed with tightness in his left shoulder.