Walker struggled during his previous stint in the big leagues, but he was recalled from Triple-A Sacramento on Monday and hasn't surrendered a run in any of his three appearances since. The 30-year-old right-hander has allowed four hits and one walk while striking out three in 3.2 innings over that three-game stretch. Overall, he owns a 6.60 ERA, 1.77 WHIP and 22:16 K:BB across 30 innings with San Francisco this season.

Walker will receive another opportunity in the major leagues after he was optioned July 9. He's been solid at Triple-A Sacramento in 12 appearances since his demotion, working to a 1.47 ERA and 0.93 with a 19:6 K:BB across 18.1 innings.