Patrick Corbin had been scheduled to start the opener, but last Saturday, he suffered a partial tear of his ulnar collateral ligament and is all but certainly lost for the 2014 season.
"This is Pat's role," Miley said. "I'm just filling in for Pat. He earned it, he deserved it and it stinks what happened to him. It stinks for our team, it stinks for him for him as an individual, but he's a strong guy -- he'll be able to overcome this and hopefully next year be back where he belongs to be."
Miley was one of Arizona's best pitchers last year, going 10-10 with a 3.55 ERA in 33 starts, a solid encore for his rookie season in which he had a 3.33 ERA and finished second in the 2012 National League Rookie of the Year Award voting.
This spring, Miley has looked sharp from his first outing.
"He got built up real quick," D-backs general manager Kevin Towers said. "Solid mechanics, his fastball, his breaking ball. I think he's in the right frame of mind right now. Pretty confident."
Arizona had little hesitation in giving Miley the Opening Day start from a baseball standpoint, but the club weas a little nervous how the 27-year-old from Loranger, La., would handle the 15-hour plane ride to Australia.
The massive 747 chartered aircraft the D-backs flew over was by far the biggest plane Miley had flown on, but it was an easier trip than he initially feared.
While a plane ride might give him some jitters, Miley is the opposite on the mound.
"Game day, he's probably as loose and comfortable as any starter I've ever been around," Towers said. "You see him on game day, and you wouldn't know he's starting."
Before the injury to Corbin, Miley was not scheduled to come to Australia with the team. Instead, he was going to stay behind with fellow starters Brandon McCarthy and Bronson Arroyo to throw in a Minor League game.
With that in mind, Miley hadn't paid much attention to all the Australia talk, or about the historic Sydney Cricket Ground, where the games will take place.
"I was interested in it, but when people were talking about the Cricket Ground, I was like, 'Whatever,'" Miley said. "I wasn't going to see it."
Yet there Miley was on the D-backs' first day in Australia, walking on the hallowed grounds. He admitted to a few butterflies and was fascinated by the history that oozes from the main pavilion, which dates back to the 19th Century.
"It's pretty legit," he said.
So is Miley, who was selected by Arizona with the 43rd overall pick in the 2008 First-Year Player Draft.
Miley's rise through the D-backs' Minor League system was methodical, and despite posting solid numbers over eight games during his Major League debut season of 2011, he was left off the team's roster for the NL Division Series.
The following spring, it didn't appear that Miley would make the team out of Spring Training. He would later joke that he had his car packed to go to Triple-A Reno when an injury to Takashi Saito gave him a spot in the bullpen.
Eventually Miley took over for Josh Collmenter in the rotation, and he wound up making the All-Star team while going 16-11 with a 3.33 ERA.
Now Miley gets to experience an Opening Day start.
"Last year when I was in the dugout, I got jitters for Ian," Miley said of Ian Kennedy, who has started the last three openers for Arizona. "I can't imagine what Ian was going through. I just closed my eyes and put myself in that position. Everybody wants to do that. It's everyone's goal to be the best they can be. It hasn't really sunk in yet."
Miley was asked if the All-Star Game experience he had two years ago would help him manage his emotions in the opener.
"Yes and no," he said. "The All-Star Game is more about enjoyment. This is a real game."
Miley then paused as he realized that the D-backs are the home team in the series.
"It just hit me that it's going to be the first pitch of the baseball season, so that's pretty cool," Miley said.
When he was a kid, did he dream that he would be doing that?
"I never thought like that," Miley said. "I just always dreamed as a kid just to be at the highest level some day. I just want to keep going and see where it takes me."
So far, it's taken Miley from a small town in Louisiana to throwing the first pitch of the Major League Baseball season in Australia.