The surgeon who operated on Mr Bolsonaro said the 63-year-old had suffered multiple internal injuries to the small and large intenstine during the attack.
"(The pain) was intolerable and it seemed like maybe something worse was happening", he said in a video from his hospital bed early Friday. Still, analysts said Thursday's attack may give his candidacy a fresh momentum.
Bolsonaro, who is known as Brazil's Donald Trump, is in stable condition, according to his son, Flavio Bolsonaro.
Bolsonaro's son, Eduardo, also a congressman, said his father had lost two liters (4.23 pints) of blood from the ordeal.
O Globo cites a source in the campaign who described the stab wound as "superficial" and expressed optimism about Bolsonaro's ability to recover and continue campaigning.
The first round of the election is October 7.
His doctor said he will be in intensive care for at least seven days, and will have to return for more surgery in coming months.
He described a painful wound, saying he had anxious about an attack on the campaign trail. "I was preparing for this sort of thing".
It was the first public opinion poll since former president and popular leftist, Luiz Inacio da Silva, was barred from running by Brazil's electoral court on September 1. Bolsonaro is now second, and while he has enthusiastic followers, his disapproval rating is higher than any other major candidate.
Bolsonaro's tiny coalition has nearly no campaign time on government-regulated candidate commercial blocs on television and radio.
His statements about women, minorities and LGBT (lesbian, gay, bisexual and transgender) people have repelled some Brazilians, but attracted many others who support his free-market policies and hard-line law-and-order stance.
Bolsonaro, a former military man and lawmaker, has been criticized for outbursts deemed racist, mysogynist and homophobic. "It's time that all presidential candidates make a declaration together to stop the aggressiveness".
Cellphone footage released by local media showed the candidate sitting on a supporter's shoulders and waving to the crowd when someone plunged a knife into his abdomen. Supporters can then be seen beating up the suspect. Other clips show supporters carrying him to a auto.
Local police in Juiz de Fora confirmed to Reuters that a suspect was in custody, but they did not identify him.
De Oliveira was beaten badly by Bolsonaro supporters after the attack.
Bolsonaro's rivals in the race expressed outrage at the attack.
That plays well with wealthier voters, but is terrifying for the one-third of Brazilians who tell pollsters they are as afraid of police as criminals.
"I can't say he was insane, but, from the way he acted, he wasn't normal, no", the relative was quoted as saying.
Supporters there included Bruno Engler, 21, who is running for a Minas Gerais state congressional seat for Bolsonaro's Social Liberal Party. Officials quoted by Reuters news agency said he appeared to be mentally disturbed.
Note, according to experts, Bolsonaro is the main contender for victory in the presidential election.