I was injured outside of work after only two weeks on the job. I think recovery may take longer than four weeks.

I started a new job working retail at a small business two weeks ago. Things were going great until I injured my back to the point of disability. I can't walk or bend down without horrible pain. I really like this job and it pays well But I am afraid to lose it because of the potential long term recovery and surgery that may happen. How do I break this news to employer and maximize the possibility of keeping my position?