88 year old father denied acute rehab. What can we do to appeal this decision?

On Monday, my father went to sleep moved the wrong way and discovered that he was almost completely paralyzed from the neck down. Tuesday, my father just had anterior cervical discectomy and fusion surgery. He is improving everyday. He had PT and OT eval- along with rehab doctor. They all decided that he should have acute therapy and he would do great! The hospital has this facility and want to move him over. However- United health care Medicare advantage has denied services. How can this be when all the doctors are saying this is what he needs? What can we do to appeal this decision?