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. We expect him to be back here."
Burgos, 24, pitched well earlier this year in 13 games with the D-backs, but Hale said he overworked him, which led to the tendinitis in his shoulder.
"It just felt like the ball wasn't going anywhere, didn't have any life," Burgos said about the injury. "I don't want to say painful, but it was like really heavy to lift my arm up or to throw the ball."
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.