Barring Herz completing crapting the bed the rest of the season, he's gonna be a rotation piece with Gray out. Honestly, its more likely that Cavalli ends up in the pen at this point. He simply can't stay healthy as a starter.
Also, for bullpen rebuild, we have more than enough options to fill in. Rutledge, Adon, Ribalta, Cate, Garcia, Brzykcy, Law, Acosta, and Cole Henry are all candidates for the pen. Tack on a couple cheap free agents, and the bullpen has plenty of depth. Where as, for starters, we are pretty lean unless they re-sign Williams or a similarly priced free agent. Unless he completely bottoms out, they simply can't afford to let Herz move to the pen just yet.