Reed was optioned to Reno on Monday when the team activated right-hander Enrique Burgos off the 15-day disabled list.
In 25 games this year Reed had a 5.92 ERA, and he lost his job as the closer in mid-May.
"He knows he's got to pitch better," manager Chip Hale said. "He's going to go down and work on his split-finger fastball. He's going to add that to his repertoire, be able to throw it in games and maybe get away with some mistakes down there where he wouldn't get away with it here. His fastball command has got to get better.