Winnie Madikizela-Mandela said the 95-year-old was not on life support but he was no longer talking "because of all the tubes that are in his mouth to clear (fluid from) the lungs".
"He can't actually articulate anything" as a result, she told The Sunday Independent newspaper.
"He communicates with the face, you see. But the doctors have told us they hope to recover his voice."
Mandela was discharged on September 1 to his home in Johannesburg's upmarket Houghton suburb after nearly three months in hospital for a lung infection.
"I have heard this nonsense that he is on life support. He is not," Madikizela-Mandela said.
Mandela is under the care of 22 doctors, and while his pneumonia has cleared, his lungs remain sensitive, she said.
"It is difficult for him," said Madikizela-Mandela. "He remains very sensitive to any germs, so he has to be kept literally sterile. The bedroom there (in Houghton) is like an ICU ward."
"He remains quite ill, but thank God the doctors were able to pull him through from that (last) infection," she said.
Mandela, who spent 27 years in apartheid jail before becoming South Africa's first black leader, has faced several health scares.
His most recent hospital stay was his longest since he walked free in 1990.
Mandela was in "an atmosphere he recognises", Madikizela-Mandela said.
"When he is very relaxed, he is fine and it has given us a lot of hope."