Long before the downfall of his mentor Carlos Ghosn, Nissan Motor Co. Chief Executive Officer Hiroto Saikawa had pushed to win more power for the Japanese carmaker in its strained alliance with Renault SA. Now he has a chance to do just that.

Saikawa, 65, has emerged as the main winner from Nissan Chairman Ghosn's shock arrest on Nov. 19 for suspected financial offenses, putting him in a position to re-balance what he and others at the Japanese company view as an increasingly lopsided partnership. Exactly how that might occur is unclear, but the idea of an outright merger between Renault and Nissan — a deal that Ghosn had advocated for over opposition from Saikawa and others — is almost certainly dead.

"It's a coup. Ghosn's era is over," said Tatsuo Yoshida, an analyst at Sawakami Asset Management and a former Nissan employee. "Saikawa is in a position to represent anti-Ghosn sentiment that's been building up for years within Nissan."