Walker will take the 25-man roster spot of right-handed pitcher Blake Parker, who was designated for assignment earlier today.
Walker, 23, was placed on the 15-day disabled list July 6 with tendinitis in his right foot. His stint on the disabled list included one rehabilitation start with AAA Tacoma on Aug. 1 vs. Albuquerque. In that game, working with a pitch-count limit, he left after 4.1 innings with an 8-2 lead: 4.1 IP, 3 H, 2 R, 2 ER, 2 BB, 1 SO, 1 HR. He threw 69 pitches, including 41 strikes.
This season Walker is 4-7 with a 3.66 ERA (35 ER, 86.0 IP) with 80 strikeouts and 18 walks in 16 starts. Among American League leaders, Walker currently ranks T5th with a 4.44 strikeout-to-walk ratio (80 K, 18 BB). Walker has twice this season equaled a career-high with 11 strikeouts in a single game (4/25 vs. HOU & 6/8 vs. CLE). Walker posted a 1.44 ERA (4 ER, 25.0 IP) in the month of April, the 3rd-best April ERA in Mariners history (min: 4 GS).
In parts of four Major League seasons with the Mariners, Walker is 18-18 with a 4.02 ERA (138 ER, 308.2 IP) in 56 career appearances/53 career starts.