The Angels assigned the two-time All-Star and former NL Rookie of the Year to Triple-A Salt Lake on Monday.
The 31-year-old Willis had a league-leading 2.56 ERA in 14 starts with the Long Island Ducks this season.
Willis hasn't pitched in the big leagues since 2011 with Cincinnati, and has struggled with numerous injuries and an anxiety disorder over the years.
He went to spring training with Philadelphia in 2012, pitched in the Baltimore's system later that year, and went to camp with the Chicago Cubs last spring.
Willis began his career with the Florida Marlins, winning a World Series ring as a rookie in 2003, and later pitched for Detroit and Arizona.