The PhD in business disciplines is structured to train students for academic research and a main part of any academic appointment is teaching. Technically, you would be eligible to teach as an Instructor with an MBA but would likely not place in a tenure track position. If you wish to expand your career possibilities in business academia, then a PhD is necessary. To apply for programs, you would need a current GMAT score, letters of recommendation from academic sources (Keep in touch with your professors!) and must explain in your application essay a desire to perform academic research. Linking your professional experience to research interests could also be helpful to your application.