Is there a replacement for metronidazole for diverticulitis?

Yes: I assume you are referring to outpatient treatment of mild diverticulitis. Flagyl is used to cover anaerobic bacteria that might be involved in the infection and i would not recommend using it alone. Antibiotic used needs to also cover gram negative aerobic bacteria, which is why two antibiotics are usually prescribed. You should discuss this with your physician.

Yes: Doxycycline combined with Cipro (ciprofloxacin) would be an option, and usually Metronidazole is used in combination with some other drug. Augmentin can also be used but is more likely to be associated with diarhea, which is not ideal.