“How did he injure his hip away from the field?”
He didn’t. It sounds like a surgeon injured him.
The surgery was related to an on-field injury. His wording in the announcement, “blindsided by an off-field injury that was beyond my control”, makes me think he is seeking some compensation, either through malpractice or other insurance that would not apply for an "on-field" injury.
Bingo. The careful wording of the public statement suggests that he was lawyered up and will eventually make a pay up demand on his surgeon and the insurers involved.