What I recommend
Change to Ceftriaxone and metronidazole.
This is a case of “Mild to Moderate”” diverticulitis. Carbapenems are almost never empirically indicated for diverticular disease and the first choice for “Severe” diverticulitis is piperacillin-tazobactam. Have a look at these therapy recommendations for diverticulitis.
In general, carbapenems should not be the first therapy for intra-abdominal infections that arise outside of hospital. It is useful to frame the choices of empiric therapy for intra-abdominal infections into 4 accending choices:
Cefazolin/metronidazole (met/cef) This is the therapy of choice for uncomplicated infections such as appendicitis in otherwise healthy people that have not been recently hospitalized.
Ceftriaxone/metronidazole The “step up” from met/cef that is appropriate for uncomplicated but more serious infections in people with underlying medical conditions.
Piperacillin/tazobactam Appropriate initial empiric therapy for people with complicated intra-abdominal infections especially those that have recently been in hospital where the likelihood of colonization with more resistant organisms is higher.
Imipenem Generally not indicated as empirical therapy. The exception is for people that have had recent infections caused by microbiologically proven resistant organisms.
More general information about carbapenems can be found here Carbapenem Notes