Gallen was diagnosed with a hairline fracture in his right forearm Tuesday.
The 25-year-old was scratched from Monday's scheduled start with lateral soreness in his right forearm, and a hairline fracture was revealed upon further examination. It's unclear how long Gallen is expected to be sidelined, though he'll apparently be able to continue playing catch at "a low stress level" from the start of his recovery. The right-hander should be considered out indefinitely until a more concrete return timeline is provided by the team, though it's safe to assume he'll be unavailable for the start of the season with Opening Day little more than one week away.