And he was phenomenal.
Ranked as Seattle's top pitching prospect by both Baseball America and MLB.com in each of the past two seasons, Walker certainly didn't look out of place on a major league mound, recording eight straight outs to begin the game before Seattle's defense betrayed him.
Stop me if you've heard that one before.
The Mariners beat the Astros 7-1, with Walker picking up his first major league victory, though he was robbed of his first career quality start, as he was pulled after only five innings of work.
Let's take a closer look at what the 21-year-old right-hander had going for him.
Command was one thing that B/R's Adam Wells pointed to as an area of concern for Walker when he delivered his scouting report for the 21-year-old on Wednesday:
The biggest issue Walker has is fastball command. He has a great, explosive heater, but can have some problems finishing his delivery out front, which causes the fastball to sail on him. He's walked 57 in 141.1 innings this season, including 27 in 57.1 Triple-A innings.
Because the velocity is so good and Walker is able to stay around the zone with the heater, he can get away with average command this season, but adjustments will have to be made in camp next season before he's ready to showcase his true talents.
Walker had command of all of his pitches in his major league debut, throwing 43 of his 70 pitches on the night for strikes.
The fact that he only walked one batter is a testament to just how locked in the 21-year-old was.
Walker didn't have his swing-and-miss stuff working for him in his major league debut, with only six of his 70 pitches on the night eliciting an actual miss.
But he kept his fastball in the low-to-mid 90's, using a 95 mile-per-hour heater to get Jason Castro to swing—and miss badly—for his first career strikeout.
While Walker wasn't blowing hitters away with pure, unadulterated heat, his pitches stayed right around where they've been all season long with Double-A Jackson and Triple-A Tacoma.
Walker's fastball has some late movement to it, but it's his secondary pitches, namely his cutter and curveball, that drive batters, well, batty.
The cutter has big-time glove-side movement and eats up bats, while his curveball, a 12-to-6 pitch that breaks over two planes, was his most impressive secondary offering on the night.
High-Pressure Situations: A+
Most veterans, much less 21-year-old rookies making their major league debuts, would have lost it in the top of the third inning, when Seattle's defense simply collapsed:
Not Walker, who brushed off the defensive miscues by his teammates and went to work against Astros second baseman Jose Altuve, getting the 2012 All-Star to hit a harmless fly ball to right field for the final out of the inning.
Under pressure, Walker didn't crack. He rose to the occasion, holding Houston to just one run on the board.
Taijuan Walker lived up to the hype, and for that, Mariners fans have to be ecstatic.
Realistically, both of the hits that Walker allowed on the night could have been scored as errors, and while he was facing the lowly Astros, Walker's stat line—five innings, two hits, no earned runs, one walk and two strikeouts—was impressive.
With Felix Hernandez and Hisashi Iwakuma already established stars at the major league level and the addition of Walker, Seattle could have as good a Top 3 in its rotation as any team in baseball next season.
He's that good—and he's only going to get better.