A man suffered a significant injury when he was struck in the head by an unknown attacker in the South of Market area Monday, police said.

The 48-year-old victim was walking in the area of Fifth and Folsom streets around 11 p.m. when he was struck in the head by an unknown goon with an unknown object. The victim was knocked to the ground, cops said. A passerby called 911.

The victim was taken to the hospital with what were believed to be life threatening injuries but he is expected to survive, police Officer spokesman Albie Esparza said.

The victim appeared to be very intoxicated police said. He did not know what happened to him or who attacked him, Esparza said.

rnagle@sfexaminer.com