Ross filled in for starter Bartolo Colon, who had just been suspended for violating baseball's substance policy, in his last appearance.
When the A's optioned him to Triple-A Sacramento the next day, he was asked to go to the bullpen.
"It will be more of a length-type situation," A's manager Bob Melvin said. "With his stuff, his best chance is in the bullpen."
He's started 13 games for the A's this year, with a 2-9 record and 6.45 ERA. Ross allowed five runs on nine hits in six innings in his last Major League outing. His next appearance will be in relief.
Ross appeared in two games, allowing four runs, three earned, on four hits and a walk in two innings of relief work with the River Cats. He was 6-2 with a 2.99 ERA in 15 games, 13 starts, overall with the River Cats.