Speaking to reporters, Trump said the vote "looked terrible" and "like another rigged election."
"I didn't take sides," Trump said when asked for his thoughts. "Elections have to be indisputable, and it looked very bad to me."
Bolsonaro, the son of former Brazilian President Jair Bolsonaro, initially refused to recognise Lula's victory, saying it was "impossible" to acknowledge the result and that the election should have been annulled. He has since said he would "organise the opposition." In a video post shared on social media on Monday, he added that he had been banned from leaving Brazil and had been "served a notice to surrender my passports."
Trump welcomed Bolsonaro to the White House earlier this year, later calling him a "smart young man who loves his country, Brazil, very much."
The Trump administration has hailed a recent wave of right-wing election victories across South America. "In South America, it was all leaning big left. It's all right now," Trump said last week. "I endorsed many of those people. Every single one that I endorsed won."