Walter Baer, chief of forensic medicine at Zurich University, said "any experienced forensic doctor would not hesitate to assume this was a case of self-infliction", but added investigations were still continuing.
The story has sparked anger in Brazil, including condemnation from Luiz Inacio Lula da Silva, the country's president.
"I think we can't accept and we can't stay quiet facing this remarkable violence against a Brazilian woman abroad," he said.
Oliveira's father has criticised those doubting his daughter's story.
"They're trying to transform the victim into a criminal. This is the tactic of a Nazi militia,'' Paulo Oliveira told the O Estado de Sao Paulo newspaper.
Earlier he said his daughter was cut about 100 times in an attack that lasted 10 minutes.
"What they did to my daughter is like a horror movie,'' he told Brazil's Globo TV.
Marco Trepp, Oliveira's fiance, said she was still terrified by the attack and suffered nightmares.
On Thursday Brazil's foreign minister called on a Swiss diplomat to share details of an investigation into the alleged incident.
Celso Amorim said Switzerland must carry out a full investigation "not only for the person attacked, but also for the good relations between Brazil and Switzerland."
The Swiss People's Party, which has taken a hard line against immigrants, condemned the attack.
"This is a country of law, where every human being deserves respect,'' Oskar Freysinger, an SVP politician, told the Associated Press news agency.
"If that really was someone from our party, we wouldn't hesitate for a second. That person would be immediately kicked out."