The 25-year-old Gallen hurt his forearm after getting jammed while taking batting practice nearly two weeks ago. Gallen said the injury doesn't affect most of his pitches, but he feels some discomfort when throwing his curveball.
D-backs manager Torey Lovullo said he wasn't sure how much time Gallen will miss. The right-hander is still allowed to play catch, and the injury will continue to be evaluated. He received two different medical opinions and both diagnosed the injury as a fracture.
Gallen was practicing his hitting because the National League will switch back to pitchers batting for the 2021 season after adopting the designated hitter rule during 2020's shortened 60-game schedule.
"It's definitely frustrating considering I'm kind of an advocate for the DH," Gallen said Monday. "I get paid to pitch, that's kind of how I look at it. It comes along with the job. I play in the National League -- you have to hit, you have to bunt, you have to do those certain things."
Gallen was the Diamondbacks' best pitcher last season, finishing with a 3-2 record, 2.75 ERA and 82 strikeouts in 72 innings. He finished ninth in the NL Cy Young Award voting.