Chavez appeared in nine games with the Blue Jays earlier this season and posted a 8.44 ERA. He received a brief shot at a starting job but a lack of command ultimately led to his demotion.
The 29-year-old spent the vast majority of his season with Triple-A Las Vegas, where he posted impressive numbers despite competing in the hitter-friendly Pacific Coast League.
Chavez went 8-5 with a 3.98 ERA in 17 starts for the 51s, and had been one of their more reliable pitchers. He was originally claimed through waivers by the Blue Jays last year after appearing in 143 career games for Pittsburgh, Atlanta and Kansas City.
The 6-foot-2 hurler filled the open spot on the A's 40-man roster that was created when Bartolo Colon was suspended 50 games after testing positive for a performance-enhancing drug.