LOS ANGELES — In 2015, Illinois native Bobby Miller was playing in a showcase for the top high school baseball players in the country when he asked his dad if they could make a stop. 

Miller had always wanted to attend a game at Dodger Stadium, and the trip to Southern California afforded the opportunity. He sat in the front row of the right-field pavilion, admiring the atmosphere, thinking to himself that playing on that field would be the ultimate dream. 

On Monday, the rookie will take the mound at Dodger Stadium for his postseason debut, entrusted by the Dodgers' coaching staff and front office to start Game 2 with his team in a 1-0 hole against the Diamondbacks in the National League Division Series. 

Miller will join Dustin May, Tony Gonsolin and Walker Buehler as the only Dodgers rookies to start a playoff game in the past six years. Unlike recent seasons, though, none of those other pitchers will be available. 

Injuries begot opportunity for Miller, who rose to the top from a talented group of prospects and rookies expected to make a leap at some point in 2023. Many were forced to contribute sooner than anticipated as a depleted Dodgers rotation suffered hit after hit. 

Now, four of the 13 pitchers on the Dodgers' NLDS roster are rookies — none more important than Miller, the 2020 first-round pick who has made tremendous strides since his days bouncing between the rotation and bullpen at Louisville. He has gained acclaim for the triple-digit heat he can conjure from his right arm, but it is his ability to command that fastball and the extensive arsenal around it that has yielded big-league success and allowed him to thrive against righties (.692 OPS) and lefties (.607) alike.

At the college ranks, Miller always possessed the ability to overpower lesser hitters. One game in particular against Valparaiso resonated with Henry Davis, the former Louisville catcher who would go on to be the top overall pick in 2021. It was 46 degrees at first pitch as Miller struck out 12 over six innings. 

"Our second baseman is wearing two sweatshirts underneath the jersey," Davis, now a rookie outfielder with the Pirates, recalled to FOX Sports, "and Bobby is sitting 101." 

But it would take more than that to survive in the big leagues. 

Davis noticed that growth on July 5. He got to Miller once for a double, but Miller struck out his ex-teammate in the other two at-bats. On one of the two strikeouts, Davis worked a full count and thought he was getting a four-seamer up … until the slider darted away from him. Before, Davis explained, the movement on Miller's pitches wasn't always so consistent. Now, he can tell it's intentional. 

"He's always obviously thrown incredibly hard, forever," Davis said. "But now you've got a guy with closer stuff that, when you look at the scouting report, he has five pitches."

Miller's curveball, changeup and sinker have been vital to combating left-handers, who are hitting under .160 against all three pitches. He has favored his four-seamer and slider more against right-handers, and while the latter has been a work in progress throughout the year as he worked to find a shape he liked, it still stood out to Davis. 

It has taken an evolution for Miller to reach this point. 

"Sometimes, I would just go up to the bullpen and throw to throw," Miller admitted. "I didn't know the whole purpose behind what I was doing that day. Now, each week, I have a purpose behind everything I do. Every bullpen I throw, every pitch I throw, there's a purpose behind everything." 

Honing his secondary pitches and learning how to sequence was only part of the growth. The bigger part was mental. 

While pitching at Louisville, Miller's favorite major leaguer to watch was Buehler. He admired the postseason standout's competitiveness on the mound and believed he possessed some of those same traits. Sometimes, though, that could also be Miller's undoing. 

Miller came out of the gates in historic fashion this year, posting a 0.78 ERA in his first four major-league starts. As he gained confidence in how his stuff played against big-league hitters, opponents began to adjust. The Giants tagged him for seven runs, then the Astros for six. In a span of eight starts from June 17 to Aug. 4, Miller logged a 6.25 ERA while going six innings or more just once. 

Too often over the past few years, Miller thought he let his emotions get to him on the mound. Frustrations would derail his performance. He lamented an inability to get ahead of hitters. The first batter would reach, and a long inning would follow. The game would speed up, his pitch count would rise, and his day would be cut short. He has learned to control that better.

"When stuff starts to go in the deep end, just not let the game speed up on you," Miller said. "Slow it down a little bit, take a quick step off the mound, deep breath, and just find something comforting mentally to cheer you up a little bit." 

He would bounce back. On his off days, the Dodgers began to have conversations in the dugout with him. They'd test him: Why did you throw a fastball in this count? Why did you go offspeed there? 

Assistant pitching coach Connor McGuiness began to notice significant growth in Miller's game management. 

"Understanding the situation, what inning he's in, how many times he's been through the order, who's on deck, what base is open," McGuiness explained to FOX Sports. "I know early on when Bobby was here, you'd kind of see him run into some trouble in that fifth, sixth inning. We're starting to see him kind of be able to break through that wall and push through." 

Coincidentally, his second half began to turn after a start against the Diamondbacks, when he shut them out for six innings on Aug. 9. Miller went at least six innings in nine of his final 10 regular-season starts, tallying a 3.25 ERA in that span. He also went from averaging 1.88 walks per game in the season's first half to 1.20 in the second. 

"You just kind of slowly chip away at it," McGuiness said. "It's a slow drip."

In the process, Miller earned his team's trust. He surpassed his innings total from any previous season, but the Dodgers decided against shutting him down. He found a groove, and a depleted pitching staff needed him. 

That remains the case in October. 

On Monday, the Dodgers will go from their most experienced pitcher to one making his playoff debut. Miller will face an Arizona offense that forced Clayton Kershaw into the shortest start of his career in Game 1. 

"I've learned a lot since I've been here," Miller said, "and I feel very ready."

Rowan Kavner covers the Dodgers and MLB as a whole for FOX Sports. He previously was the Dodgers' editor of digital and print publications. Follow him on Twitter at @RowanKavner.