The man was not able to tell investigators what he was attacked by, but officials say his injuries suggest he was attacked by a cougar at a homeless encampment.
Jorge Maradiaga says the man knocked on his door, begging for help.
"You could see his teeth marks," said Maradiaga. "The ambulance pulled up, and they started cutting his shirt and that's when you see all the claw marks and that's when they said this was an animal attack."
Authorities believe the attack occured either late Thursday or early Friday near Highway 74, west of the 215 Freeway.
The unidentifed man was transported to a nearby hospital with lacerations, puncture wounds and bite marks at the base of the skull.
Crews were searching overnight for any signs of the mountain lion. Baited traps were also set, but investigators have yet to find the animal. The traps have since been removed, and crews are trying other tracking techniques.
The victim remains in critical condition, but he is expected to survive, officials said.
California Department of Fish and Wildlife officials said they collected DNA from the victim to match the lion when it is captured. It will then be "destroyed," according to CDFW.
"The first priority of any law enforcement agency is the safety of the public and we are doing everything we can to find and capture this animal before it can harm anyone else," said CDFW Assistant Chief Dan Sforza. "We are asking nearby residents to be aware there is a lion in the area and to be careful with their pets and children."
Investigators believe the mountain lion is between 18 months and 2 years old.