Suprvsr told HR I needed FMLA before I decided 2 have surgery. What can I do?

My supervisor requested FMLA from HR seven days before my doctors appt to discuss surgical and non-surgical options. My doctor and I decided to definitely schedule surgery but now I can't meet the 15 day deadline to submit the medical certification because it's in 4 days. I emailed HR and told them I wouldn't have requested it yet and my supervisor jumped the gun but HR said the 15 days to submit the paperwork is standard federal law from the time HR is notified. I didn't notify them nor did I definitely know I was having surgery. HR said my request will be denied if I dont make the deadline. My doctor outsources the certification to a third party. Its also the holidays. I have no control over when the company completes the certification. I feel like this is unfair. What can I do?