"He said [to me] it's a slow recovery process not without risk. He's 80 years old," Chavez said, referring to a telephone conversation with Castro about a week ago.
The Venezuelan president regards Castro as a political mentor and accused the US of exaggerating Castro's illness.
"The empire [the US] is bent on killing off Fidel Castro," he said.
The US media has quoted American doctors as saying that Tuesday's report in El Pais, a Spanish newspaper, suggested Castro had received questionable or even botched care.
El Pais reported that Castro was in serious condition and being fed intravenously.
It said he had undergone three failed operations on his large intestine for diverticulitis, or inflamed bulge in the intestine.
Castro, who has led Cuba since 1959, has not been seen in public since July, fueling speculation he is so ill he may never return to power. He has handed over power to his brother Raul, Cuba's minister of defence.