It's quite the impressive feat for Walker, who was selected in the first round of the 2010 draft. But the talented right-hander is a bit behind the pace of teammate Felix Hernandez, who pitched in his first major league game at age 19.
Walker, who started the season at Double-A Jackson before being promoted to Triple-A Tacoma, got a welcome and a hug from Hernandez on Thursday. Hernandez hadn't had time to chat with Walker yet, but the ace was ready if the youngster asked for any advice.
"You've just got to be relaxed, and after you throw the first pitch everything's going to go away," Hernandez said.
Walker, who is thought to be among the top prospects in the sport, says he knows he'll be nervous in the first inning, but hopes he calms down after that.
He went 4-7 with a 2.46 ERA in 14 starts at Double A and 5-3 with a 3.61 ERA in 11 starts in Triple A. He was dominant in his last two starts, going 2-0 with a sparkling 0.82 ERA.
General manager Jack Zduriencik said there were many factors that went in to bringing him up, but discussions with minor league pitching coaches convinced him Walker was ready.
"This kid's got great stuff, he's a fabulous athlete," Zduriencik said. "He's done a really great job maturing, growing up, handling the things given to him this year and you just make a decision on what you think is best for him and the organization."
Walker went through some struggles last season in Double A, and said he spent the early part of this season working on refining his pitches.
"I worked on fastball command and my secondary pitches," he said. "I spent the first month just really, really trying to work on them hard and I struggled pretty bad. But it wasn't about the wins and losses, it was mostly about just working on the pitches and working on the fastball command."
The work paid off and Walker steadily improved throughout the season. Manager Eric Wedge loves Walker's confidence and said he's mature beyond his years.
Wedge's expectations for Friday are simple.
"I just want him to go out there and ... be himself, enjoy the experience and have fun," Wedge said. "Hopefully he'll be able to allow himself to relax and just have fun with it. That's the best way to have early success is just to relax and have fun with it and just really enjoy the moment."
Walker has family flying in from California and driving in from Louisiana for Friday's game. He expects to have a personal cheering section of about 30 people when he takes the mound.
"I'm glad I get the opportunity to come out and pitch in the big leagues and have my family out there," he said. "A lot of them haven't got to see me pitch yet, so it's going to be pretty exciting. You'll probably hear my mom; she'll be the loudest person in the stands. You guys will definitely hear her."
Seeing Walker prepare for his first start caused Hernandez to reflect on his debut in Detroit eight years ago. He said it was a special moment and he hopes Walker's will be just as memorable.
"He's pretty good," Hernandez said. "I can't wait to see what he does."
The Astros (44-89) will counter Walker with Brad Peacock (3-4, 5.67), who is 2-1 with a 2.88 ERA in four starts since being recalled from Triple-A Oklahoma City.
The rookie right-hander struggled in his latest outing, however, allowing four runs and two homers in 5 2-3 innings of Saturday's 8-5 win over Toronto.
Peacock beat Seattle 8-3 with five innings of three-run ball April 10 before surrendering seven runs and three homers while being pulled with one out in the fifth inning of a 7-1 loss in Houston 12 days later.
Seattle (60-73) ended a six-game losing streak in the opener of this four-game set Thursday night, winning 3-2 to send Houston to its third straight defeat.