Walker, 21, is the organization's top pitching prospect and was listed before the season by MLB.com as the No. 5 prospect in baseball.
The official roster move will be made Friday before the 5:10 p.m. game at Minute Maid Park.
Walker will become the fifth-youngest Mariner to make his debut as a starting pitcher.
He has split this season between Class AA Jackson and Class AAA Tacoma. In 11 appearances with the Rainiers, he was 5-3 with a 3.61 earned-run average with 64 strikeouts over 571/3 innings. He pitched in the All-Star Futures Game and was a Southern League All-Star.
Walker is 6-foot-4, 210 pounds and a native of Yucaipa, Calif. He originally was taken by Seattle in the supplemental first round (43rd overall) of the 2010 amateur draft.
Even before the announcement, manager Eric Wedge had ticked off a list of young Mariners he believes will form the core of the club for seasons to come.
"You look at all of our young players, there are eight or nine of them -- seven or eight of them -- that are going to be a part of this," he said. "Especially once you get (catcher Mike) Zunino back. You've got (first baseman Justin) Smoak, (second baseman Nick) Franklin, (shortstop Brad) Miller, (third baseman Kyle) Seager, (infield/outfielder Dustin) Ackley and (outfielder Michael) Saunders. You've talking about some guys that you can move forward or make some good progress. And all the young arms in the bullpen -- especially when you had (Stephen) Pryor -- with (Felix) Hernandez and (Hisashi) Iwakuma up top, you probably go out and get somebody in the middle there, and you've got a couple of young kids in the back end who are going to fight for it."
Within that core, Wedge said he sees a team that is more athletic, faster and more versatile than the 2013 Mariners.
"I want to have that to be an option where you can steal more, you can hit-and-run more, you can use the bunt game a little bit more: whether it be for a hit or a safety squeeze or a regular squeeze," he said. "I think we're going to have some guys that can do those things if we can stay healthy. You look at Franklin, you look at Miller, Seager to a point; but Ackley and Saunders, those are good things. You have to get rid of some of these cloggers: You can have one or two, but you can't have three or four."
As for leadership, Wedge sees that potential in some of the young guys, too. But he added that the leadership bar wasn't set as high by this season's veterans as he would have liked.
"I think they're going to get to the point where they hold each other accountable -- ride each other," Wedge said. "I think there have been some positives with the veterans with that this year; also disappointments with the veterans regarding that this year, and I don't mind saying it. It's tough to find leadership in people. Everybody wants to be liked, and everybody's afraid of confrontation. Show me a leader (who is like that) and I'll show you a bad leader."
On Wednesday, Dustin Ackley extended his hitting streak to six games. He is batting .406 in August. ... Raul Ibanez drove in his first run at home since July 14. ... Kyle Seager hit his 21st home run of the season, establishing a career high. He also leads the team in extra-base hits with 51.
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