Hospital officials indicated the man may have suffered a head injury as the result of a fall.
Officials declined to release the man's name, but said more information about his condition may become available later in the day.
The man, believed to be in his 20s or 30s, was found in the backyard of a building on Natoma Street between Sixth and Seventh streets at about 8 a.m. on June 12, according to hospital officials.
