The 6-foot-3 Iowa native has been nearly untouchable in the league this season. Through four starts, Keller is 2-0 with a 1.72 ERA and batters are only hitting .151 against him.
He has pitched at least three innings in each of his starts and has allowed only three earned runs and four walks in 15 2/3 innings.
Part of his success is coming from the fine-tuning of his secondary pitches, which he said has been a focal point since arriving in Arizona.
"Changeup has just come a long way with depth and taking speed off. Knowing when to use it early in the counts and using it when I'm behind in the counts too," Keller said. "Curveballs, the same thing. It's just been a huge learning experience for me."
Keller said he also often picks the brains of teammates and coaches.
"Just being able to get their experiences and know how to use what I have in the game," Keller said. "Talking to other players has been a big thing for me."
Success is no new theme for Keller. Since being taken in the second round of the 2014 Draft by the Pirates, he owns a combined 2.79 ERA across the Class A and Double-A levels.
In 2017, he threw 116 total innings with a respectable 3.03 ERA for Class A Advanced Bradenton, Class A Short Season West Virginia and Double-A Altoona. In six starts with the Curve, batters hit a measly .197 against him. Keller even helped the Curve win the Eastern League title by striking out eight over 7 1/3 innings and allowing only two runs in the series-clincher.
Keller is happy with the way he has performed up to this point and believes the stint in the AFL will definitely benefit his game.
"Just being able to use all of my pitches and going up against the best competition and see where I stand, I think it's been a really good experience for me," Keller said. "I think I've handled it pretty well."
Reaching the Majors may not be too far off for the 21-year-old given his dominant play in the Minors and high prospect ranking, but when that day comes, it will mean the world to Keller.
"It would just be an unbelievable experience for me and my family," Keller said. "Just to say that I was one of the few that's ever made it, would just be an awesome experience."
Kyle Wilcox is a journalism graduate student at Arizona State University. This story is part of a partnership between MLB.com and ASU's Walter Cronkite School of Journalism and Mass Communication. This story was not subject to the approval of Major League Baseball or its clubs.