Must the signature of an actual doctor appear on FMLA leave certification?

Q. An employee who is seeking FMLA leave has submitted the required paperwork from his doctor’s office, but the doctor’s staff assistant signed it. We asked our employee about the paperwork, and he told us that he went to his physician’s office and asked an assistant to fill out the certification. The doctor was not there when our employee took in the paperwork, and he admits that he had never seen the assistant before. Nonetheless, our employee is telling us that we have to accept the certification because someone at his doctor’s office signed it. Please tell me that at a minimum the physician has to know and approve of this certification ...