Saikawa may have a more personal reason to distance himself from Ghosn.
Analysts say the Nissan chief runs a risk of being entangled in the criminal investigations swirling around his predecessor and disgraced board member Greg Kelly. The latter, an American executive long involved in human-resources matters, was also detained by authorities and under investigation as Ghosn's co-conspirator.
Nissan stripped Kelly of his status as a representative director last week.
Although Ghosn was removed as chairman and both men lost their standing as representative directors, each retains a seat on the board as a standard director. That status can be revoked only through a shareholder vote, something Nissan's board is expected to pursue.
Details of the criminal case are fuzzy. But Nissan accuses Ghosn of three financial misdeeds.
Ghosn allegedly underreported his income in official stock market filings, diverted corporate investment funds for personal use and misused company expenses.
Japanese media say the misdeeds began around 2011 and spanned five years. Commentators want to know why Nissan, and Saikawa as a board member during that time, didn't spot something. Japanese media reported last week that prosecutors planned to meet with Saikawa to ask him the same thing.
"There is the possibility that Saikawa and other board members might be fired," said Koji Endo, a senior auto analyst at SBI Securities Co. "Why wasn't someone aware of the misconduct?"
Saikawa's denunciations of Ghosn were all the more shocking in Japan because Saikawa was long seen as Ghosn's right-hand man.
Ghosn parachuted in as the Renault-installed "cost-killer" to revive the struggling automaker through fiscal discipline. But it was Saikawa who won Ghosn's trust as the enforcer who helped break the keiretsu of affiliated suppliers in the early days of the revival. And later, as chief competitive officer, he helped Ghosn chart the company's course.
"We all thought Saikawa was on Ghosn's side," Endo said. "He was promoted to president only because he was a yes man to Ghosn. But at the same time there was a growing faction of people even on Ghosn's team who apparently didn't like his management approach."
Saikawa said a Nissan whistleblower called out Ghosn's misdeeds, sparking a months-long internal probe. He declined to offer details, citing the ongoing criminal investigation.
Japanese media have since suggested two whistleblowers. National broadcaster NHK identified one tipster as a non-Japanese executive working in Nissan's legal department. The person shared information with prosecutors in return for lenience under a plea-bargain system adopted by Japan just this year, NHK said.
The person was involved in the purchase of overseas properties for Ghosn's exclusive use with company funds funneled through a subsidiary in the Netherlands, NHK said.
Japan previously had no plea-bargain system, and this was just the second time the legal maneuver had been exercised since the rules were rewritten in June, the report added.
Saikawa was co-CEO with Ghosn during a one-year transition before taking control as solo CEO last year. But since then, Saikawa's tenure has been besieged by scandal.
The first crisis erupted late last year when Nissan disclosed it had been conducting faulty final inspections of vehicles at assembly plants throughout Japan.
That triggered the recall of more than 1.2 million vehicles here, a callback of virtually every passenger car the company produced for sale in Japan over the previous three years.
More inspection misconduct was uncovered this year, and Nissan is on an embarrassing regimen of filing reports to Japanese regulators on its progress in preventing a relapse.
And now come the shocking allegations of malfeasance at Nissan's highest echelons.
Hiroshige Seko, head of Japan's powerful Ministry of Economy, Trade and Industry, and Transportation Minister Keiichi Ishii warned Nissan to improve corporate governance.
Meanwhile, Nissan Senior Vice President Hitoshi Kawaguchi was dispatched to Tokyo to apologize in person at the prime minister's office for the company's ignominious state of affairs.
Some analysts say the whirlwind of uncertainty may create a need for Saikawa to stay at the helm until the storm passes. Saikawa is Ghosn's elder by a few months, but Nissan has neither a limit on the number of terms a CEO may serve nor a mandatory retirement age.
"He is not that old by Japanese standards," said Yoshida, the Sawakami analyst. "And he has the chance to take full control of the company. Saikawa is expected to be here another couple of years."
Naoto Okamura contributed to this report.