Cueto was placed on the injured list for unspecified reasons Monday.
The team didn't supply an injury designation or specify which list Cueto was placed on, which often means it's related to COVID-19. The veteran right-hander was poised to start Monday's series opener versus Milwaukee, but Jose Alvarez will now start a bullpen game for San Francisco. Assuming Cueto is on the COVID-19 IL, he'll be able to be activated once he clears the health and safety protocols.