The following information is on the Kellogg website:

Ideally, one of your letters of recommendation, or Career Progress Surveys, should be written by your current supervisor or manager. The second letter of recommendation also should be from someone who can objectively evaluate your professional performance as well as managerial and leadership potential, such as a former supervisor, previous employer, client, etc. If you are unable to ask a current supervisor or manager for a letter of recommendation, please include a brief statement regarding your choice in the Additional Information section of Part 2 of the application.

You should feel free to call the admissions office for additional details. It is better to ask for specific information than the submit subpar LORs.