Seattle Mariners right-hander Taijuan Walker began the 2013 season back in Double-A Jackson where he spent all of 2012 because he needed to improve in several areas and the club correctly deemed the Southern League as the best place for him to take on such tasks. Tuesday, the 20-year-old showed how far he’s come in only four months and why he’s among the very top pitching prospects in all of baseball. Walker, a sandwich-round selection in 2010 (No. 43 overall), used his stuff, poise beyond his years and a maturity level necessary for the circumstances to toss six shutout frames at the Fresno Grizzlies, allowing three singles and two walks, while striking out four. He threw 87 pitches, 56 for strikes, and for all 87 he appeared to be every bit the phenom he’s been cracked up to be since he burst onto the scene three summers ago as a raw 17-year-old. The 6-foot-4, 210-pound Walker featured a four-seam fastball that sat 93-95 mph and touched 97, and came to the plate with ferocious downward plane, creating a very heavy ball, one extremely difficult to square up for hitters. He commanded the pitch down all night, missing only below the zone, not up, with some occasional arm side run and natural sink. He worked his 90-93 mph cutter in often, helping set up a much-improved 71-74 mph curveball that froze batters all night. He sawed off several bats with the four-seamer and cutter and used the curveball with two strikes or to keep the Fresno lineup off balance. He tossed a half-dozen or so below-average changeups in the mid-80s, but kept it down or away from left-handed batters and one in particular showed some fade, though each lacked sink. He maintained good arm speed on the pitch, but it’s still a ways from being more than a show-me offering. Walker’s velocity didn’t dip much from the stretch — he sat 93-94 with runners on — and did not let some bad luck get to him. There were 4-6 borderline pitches on which he did not get the call from the home plate umpire, but he went back to work, rather than allowing it to get to him and affect his approach. The four-seamer induced ground balls and the cutter forced poor contact that resulted in shallow fly ball outs and pop ups, and he did miss some bats with both pitches. His curveball is a full grade better today than in spring training or at any point a year ago, and he’s taken well to the cutter and seems to know how to use it. Walker’s delivery was very consistent with one exception: He drops his arm slot slightly and opens up early on the curveball, which could be an issue against better hitters and it’s something he’ll need to fix for general consistency purposes, if nothing else, but big leaguers will see it and exploit it. Walker’s velocity comes free and easy without much effort, which bodes well for his ability to hold that velocity deep — which he did through pitch 87 Tuesday and has done through the low-100s in the past — and if his high three-quarter arm slot can be maintained with the curveball, there’s no reason to believe he won’t max out and become a No. 1 starter. There may even be ace material here, though we’re likely a few years away from that kind of command from Walker. I came into this start with one objective, which was to remain as objective as possible. Even doing so, it’s difficult to suggest Walker is on a path that will lead to anything but future stardom. That isn’t likely to occur in 2013, and while he’s likely to see the big-league mound in 2014, he may not settle in and succeed at a high level right away. His stuff, athleticism and his apparent acumen for progress may prove me wrong there, however. He was very crude coming out of high school and in less than three years has gone from high-upside project to elite pitching prospect on the doorstep of the majors. He’s raised the ceiling on all of his grades since then — here’s my 2010 draft-day scouting report on Walker for subscribers — and now the sky is the limit. After four innings, M’s scouting director Tom McNamara tapped me on the shoulder. I looked back and he gave me a look as if to ask, “so, what do you think so far?” My response? “I’d be excited if I were you.” Taijuan Walker is a phenom, and it may not be long before he’s toeing the rubber at Safeco looking to back up a Felix Hernandez gem with one of his own.