Geren said the move was made to limit the innings of his young pitching staff, which includes rookie right-handers Trevor Cahill and Vin Mazzaro and rookie lefties Brett Anderson and Gio Gonzalez.
"It'll give everybody some extra rest," Geren said. "We'll go that avenue through September with an extra pitcher and some off-days to give some of our young guys some rest."
The right-handed Gonzalez has come out of the bullpen in his past 19 appearances with the A's, and he has a 5.20 ERA in those outings. Gonzalez made two starts in May after beginning the season in Triple-A, and he was 0-1 with a 5.40 ERA in those games.
The move also shuffles the A's scheduled rotation, as Gonzalez will start Tuesday, followed by veteran Brett Tomko, Cahill, Mazzaro, Anderson and Gio Gonzalez.
But Geren said the order is subject to change based on how the pitchers are feeling, given the flexibility of a six-man rotation.
"We've put three or four models together, which is nice," Geren said. "It gives us flexibility to give these guys the amount of innings we'd like and for them to finish strong."
Rhett Bollinger is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.