OP, you have to understand that the vast majority of LORs are saying that the applicant walks on water and is a veritable saint in terms of good character. So when an adcom sees a LOR that is even lukewarm, that raises a red flag. Thus, you should not, under any circumstances, ask someone to write you a LOR unless you are confident that it will be a strong LOR. In your case, that means no LOR from your advisor. It may come up at an interview that you didn't get one from him, and if it does, be prepared with a bland answer that doesn't badmouth your advisor. Such as, you could say how you spent a lot more time working directly with the postdoc-cum-faculty, and therefore this person was much better able to assess your qualifications for medical school.