His wound was severe enough that his son said he was unlikely to be able to return to campaigning before the October 7 vote.
Bolsonaro, a 63-year-old congressman, was knifed in the stomach on Friday while being carried atop supporters' shoulders in a street rally on Thursday and was being treated at a Sao Paulo hospital.
A Tweet posted on Bolsonaro's verified account said he was "doing well and recuperating."
However, Flavio Bolsonaro, Jair's son, said in a video on his verified Facebook page Friday afternoon that his father was in a "delicate situation and has trouble speaking."
"He is recuperating and he probably will not be able to head out into the streets in this campaign," said Flavio Bolsonaro. "He cannot go to the streets, but we can."
A spokesman for the Bolsonaro campaign did not immediately return a request for comment.
The attack further clouds Brazil's most unpredictable election in three decades. Corruption investigations have jailed scores of powerful businessmen and politicians in recent years, and alienated infuriated voters.
Bolsonaro has for years angered many Brazilians with extreme statements, but is also seen by his many supporters as a politically incorrect gust of fresh air in a rotten system.
He has repeatedly said the country's notoriously violent police should increase their killing of suspected drug gang members and armed criminals.
That plays well with wealthier voters, but is terrifying for the 50 percent of Brazilians who said in a 2017 Datafolha poll they feared being victims of police violence.
Surveys consistently give Bolsonaro around 22% in simulated first-round votes. However, those polls find he would badly lose to most rivals in the likely event of a runoff, which takes place if no candidate wins a majority in the first ballot.
Bolsonaro was stabbed while being carried on someone's shoulders in a crowd of cheering supporters in the city of Juiz de Fora. TV pictures showed him screaming in pain, then falling backward into the arms of those around him.
Police video taken at a precinct showed suspect Adelio Bispo de Oliveira telling police he had been ordered by God to carry out the attack.
Speaking earlier in an online video from hospital in Juiz de Fora, Bolsonaro said the pain of the attack at first was like being hit by a football.
"It was intolerable and it seemed like maybe something worse was happening," he said, talking in a weak, raspy voice. "I was preparing for this sort of thing. You run risks."
Bolsonaro likely needs to spend at least a week in the hospital. Being unable to campaign any more would seriously damage his run.
His tiny coalition has almost no campaign time on government-regulated candidate commercial blocs on television and radio. He must rely on social media and, until now, raucous rallies around the country to drum up support.
Running as the law-and-order candidate, Bolsonaro has positioned himself as the anti-politician, though he has spent nearly three decades in Congress.
He has long espoused taking a radical stance on public security in Brazil, which has more homicides than any other country, according to United Nations statistics, and has openly praised Brazil's military dictatorship, which he has said should have killed more people.
