The Giants selected Walker's contract from Triple-A Sacramento on Friday.
Walker will join the Giants ahead of their three-game series with the Marlins. The right-hander has produced an impressive 0.89 ERA and 0.84 WHIP with 23 strikeouts over 20.1 inning s in 15 appearances, including three starts, with Sacramento this season. Walker will help replace Ross Stripling (back), who was placed on the 15-day injured list Friday. This will be Walker's first career stint in the big leagues and he could be asked to make a spot start or two.