Gallen (shoulder) will make his Cactus League debut Wednesday against the Rockies, Steve Gilbert of MLB.com reports.
Gallen has been behind since the start of camp after battling shoulder bursitis over the offseason. He's not all that far behind, however, as been throwing off a mound for two weeks. He's unlikely to be built up to a full starter's workload by Opening Day given his late start, but it's unclear if that means the Diamondbacks will send him to the injured list or simply have him finish ramping up while on the active roster.