With one out in the inning, Walker threw a pitch that ran the count to 3-2 on Chicago's Adam LaRoche and then took a step and immediately sat down and grabbed the upper portion of his right leg. Walker, who seemed to sustain the injury on the previous pitch, walked off the field under his own power after being examined by a team trainer.
Walker allowed only three hits in his 6 1/3 innings, striking out five. He left with the Mariners leading the White Sox 2-0.
"I just cramped up,'' said Walker, who was initially scared by the injury. "(My leg) gave out on me.''
Carson Smith came on and completed the walk to LaRoche that was charged to Walker.
Information from The Associated Press was used in this report.