Walker had a 5-2 lead in the fourth when he gave up a one-out homer to Steve Pearce. After Corey Dickerson followed with a triple off the left-field wall, trainer Rick Griffin, manager Scott Servais and pitching coach Mel Stottlemyre Jr. went to the mound, and Walker was taken out after a brief conversation.
Walker is 3-6 with a 3.69 ERA and had thrown eight scoreless innings with a career-high-tying 11 strikeouts with no walks in his previous outing, a 5-0 victory over the Indians.
The Mariners are already without ace Felix Hernandez, who has been on the 15-day disabled list since May 29 with a strained right calf and isn't expected back until sometime in July.