Mariners righthander Taijuan Walker, who is rated as high as the fifth best prospect in baseball, is set to make his highly anticipated major league debut tonight at Minute Maid Park against the Astros. Walker, 21, was a combined 9-10 between Class AA Jackson (4-7, 2.46 ERA) and Class AAA Tacoma (5-3, 3.61 ERA) this season, but he was 2-0 with an 0.82 ERA over his last two starts before earning a call to the majors. In those games against Colorado and Tucson he scattered a combined nine hits and six walks with 15 strikeouts while giving up only one run through 11 innings, earning the most recent Pacific Coast [...]