Called up from Triple-A Reno earlier this month, the 25-year-old out of the small Iowa town of Cherokee, made five relief appearances and allowed just one run.
When Zack Greinke came down with a stiff shoulder following his start last week in San Diego, the team decided to let Koch start Tuesday and this Sunday's season finale as well.
"A very impressive outing, but not a surprise," D-backs manager Chip Hale said. "This is a guy who's already shown in the relief appearances we've given him that he's not afraid. He goes right at guys. His ball moves, he cuts it, he sinks it, his slider was good tonight and he threw some changeups. He was around the zone, he elevated when he had to, jammed some guys."
The only thing that kept Koch from being perfect through the first five innings was when he hit Jayson Werth with a pitch in the first inning.
Jose Lobaton opened the sixth with a single to center after Koch was visited by the training staff to check on a blister that was developing on his middle finger.
Koch then walked pinch-hitter Wilmer Difo and Hale decided to pull him because he was at 69 pitches and the team did not want him to throw more than 75.
"I was just trying to get quick outs," Koch said. "I knew I was on a pitch limit, so I was trying to make the most of the pitches I had and try to get quick ground-ball outs."
The fact that he was throwing a no-hitter did cross his mind.
"It did a little bit after the fourth," he said. "I said, 'Let's just keep trying to get quick outs.' If I keep ball down -- I got it up a little in the third and fourth innings, but my mindset was just to stay focused and keep the ball down."
Between now and Sunday, Koch will get treatment for the blister to make sure it isn't an issue.
The pair of starts he has in the season's final weeks are important for Koch because they are his opportunity to show the D-backs he deserves to be considered a rotation candidate when Spring Training rolls around.