The attorney, Yury Schmidt, said on Saturday that Khodorkovsky had been assaulted by another prisoner with a handmade knife sometime between Thursday night and Friday morning.
He also accused prison authorities of covering up the incident.
The billionaire tycoon, once Russia’s richest man, is serving an eight-year sentence for tax evasion and fraud in a Siberian prison camp.
The Yukos oil empire he founded was carved up by the state.
In a statement posted on Khodorkovsky’s website, Schmidt said his client woke up in the middle of the night with his face covered in blood.
Khodorkovsky, imprisoned since October, required stitches and was recovering in the infirmary wing of the prison camp in Krasnokamensk, the lawyer said.
Another Khodorkovsky lawyer, Natalya Terekhova, said he had been stabbed in the left nostril, the Gazeta.ru news website reported.
The Federal Prison Service, however, downplayed the incident, saying its preliminary information was that Khodorkovsky suffered scratches to his nose while arguing with a fellow convict, the Interfax news agency reported.
“Khodorkovsky’s defence team has no illusions about the real organisers of this crime and is determined in accordance with the law to obtain all the details of what happened”
Schmidt said prison authorities gave no indication they planned to open a criminal case into the assault.
He said this showed “that contrary to official assurances, Khodorkovsky has fewer rights than other ordinary prisoners and he is being singled out”.
“Khodorkovsky’s defence team has no illusions about the real organisers of this crime and is determined in accordance with the law to obtain all the details of what happened,” he said.
Terekhova, who practises in the Siberian region of Chita where the prison is located, said the assailant was a 23-year-old cellmate called Kuchma who was now being held in solitary confinement, according to Gazeta.ru.
The aim was not to kill Khodorkovsky but probably to disfigure him, Gazeta.ru quoted his lawyers as saying.
Terekhova is to meet with Khodorkovsky for the first time since the assault on Monday.
Khodorkovsky had declined to submit a complaint because he believed Kuchma was not in full possession of his senses, Terekhova was quoted as saying by Gazeta.ru.
Khodorkovsky, 42, was convicted in May 2005 after a trial seen as punishment for his political ambitions and part of a state drive for control of the crucial oil industry.