His heart stopped briefly overnight and he is being kept alive with a respirator and other machines, while doctors evaluate whether to declare him brain dead, they said yesterday.
"He is in a deep coma," said Ernesto Torres, the director of the Centro Medico hospital in Rio Piedras, Puerto Rico where Camacho was taken after Tuesday's shooting.
"The bullet damaged three arteries that run through the neck and as a result blood is not flowing well to his brain," he said.
Torres said the boxer was not responding to either pain or medicine. "Macho's situation is very delicate. The prognosis is very poor," he said.
His mother, Maria Matias, arrived from New York early yesterday. The Spanish-language TV channel Univision said his brother and children were also expected, as the family decides whether to disconnect an artificial respirator.
"His mother already knows that the outlook is not good, and we told her to prepare for the worst scenario. We will continue to do tests tonight and tomorrow morning," Torres said.
"But we have to stress that Macho Camacho is breathing with a respirator and his blood pressure and pulse are being maintained by machines."
Camacho, 50, was being driven in a car when he was hit by gunshot from another vehicle outside a liquor store around 7:00 pm local time, San Juan police said.
The driver, Alberto Mojica Moreno, 49, was killed in the shooting.
It was unclear if they were targeted or simply caught in a random act of violence.
Television images and photographs circulated on Twitter showed the ex-boxer on a hospital gurney, with an oxygen mask and tubes, covered by blood-soaked covers. It said he was unconscious.
Torres said Camacho's heart stopped at 4:15 am (1345 IST).
"We were able to pull him back with drugs. We gave him drugs and his heart started beating again," he said, adding that neurologists said they "cannot do anything for him."
"This is a process and we have to wait for two different indicators to declare brain death," he added.
Camacho was one of the most colourful boxers of the 1980s.