An officer who was looking for an assault victim nearby Monday night heard the man screaming for help.
Police supported him to prevent further injuries until medics arrived and took him to a hospital, where he was in serious condition in intensive care on Tuesday.
Police spokeswoman Renee Witt wrote in a department Web site posting that officers thought the man might have been involved in the reported assault, but he insisted he was just a ninja trying to clear a 4- to 5-foot-tall fence.
Witt says the man was "overconfident in his abilities," and that alcohol likely played a role.
His name was not released.