Koch was recalled from Triple-A Reno on Saturday.
The right-hander enjoyed a sizzling start to the 2018 season, spinning a 2.59 ERA in his first five starts, but Koch soon fell apart afterward with a 5.80 ERA in the following nine outings and lost his rotation spot in late June. He'll return to the majors as a depth piece in the bullpen with potential to step in for an emergency start.