Koch will be recalled from Triple-A Reno to start Friday against the Padres in place of the injured Taijuan Walker (elbow).
With Walker expected to miss the remainder of the season thanks to a torn UCL, Koch will get the first shot to fill in for him in the big-league rotation. The 27-year-old spent some time with the Diamondbacks earlier in the season in a long-relief role, tossing a pair of scoreless innings while striking out three batters. He also impressed in his only outing with Triple-A Reno, allowing just three hits during six scoreless innings. That said, Koch struggled mightily across 10 starts with Reno in 2017, posting an 8.40 ERA and 1.84 WHIP across 45 innings, so it would be unwise to expect much in his first big-league start since 2016.