Walker, who is the top prospect in the Mariners system and No. 4 overall in baseball, was 9-10 with a 2.93 ERA across Double-A and Triple-A this year.
So what can fans expect from Walker in his debut?
Timeline of His Progress
Walker was selected 43rd overall in the 2010 MLB Draft out of Yucaipa (Calif.) High School.
He made four appearances for the Arizona League (Rookie) Mariners, going 1-1 with a 1.29 ERA in seven innings. Walker then spent all of 2011 at Class-A Clinton where he went 6-5 with a 2.89 ERA and 113 strikeouts in 96.2 innings.
He spent the following year at Double-A Jackson, where he went 7-10 with a 4.69 ERA and 118 strikeouts in 126.2 innings. Because of those struggles, he started this year back in Jackson and found success before being promoted after 14 starts.
The Mariners have been careful with his progress, noting he's still only 21.
According to scoutingbook.com, Walker works with a 94 mph fastball and an 82 mph changeup, among other pitches.
Walker works mainly with a 94mph darting fastball that has great late movement, and when he mixes in a sometimes-effective straight change at 82mph, the fastball is nearly unhittable. His breaking pitch is a slurvy curve that isn't yet ready for regular use, but he'll have time to develop.
Baseball Prospect Nation believes he's a "true front of the rotation monster."
If the change-up leaps forward to be an average pitch there won’t be much left to stop him. The ace tag is thrown around too often by fans but Walker has everything needed to develop into a legitimate top of the rotation stud.
If what the scouts are saying is true, Walker will eventually become a stud in the big leagues.
What Fans are Saying
A lot of fans are excited about Walker making his debut. Here's just a few of their reactions:
What to Expect
There's definite excitement in Seattle.
Add Walker to Felix Hernandez and Hisashi Iwakuma, and you have a formidable trio. Throw in Danny Hultzen and James Paxton next year, and you have the best young rotation in the baseball.
Expect the Mariners to allow him to get his feet wet this year. Starting with the Astros (and if he maintains his spot every fifth day), Walker will also face the Royals (twice), Astros again, Cardinals and Angels.
Outside of the Cardinals, I'd say that's a pretty good way to get your feet wet.
He'll likely go five or six innings (at most) in his first few starts, but by the end of the year, the Mariners could let him go seven or eight to see what he's got.
It may not amount to a hill of beans this year, but Walker's promotion is the beginning of something good in Seattle.