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.