Marseille president Vincent Labrune declared he was left "dumbfounded" by Marcelo Bielsa's decision to quit the Ligue 1 club.
Bielsa announced his resignation at his post-match media conference on Saturday following Marseille's 1-0 loss at home to Caen in their opening match of the league season.
It ended a typically dramatic 15-month tenure for the man known as 'El Loco', with Labrune struggling to rationalise Bielsa's announcement with their recent meetings regarding a contract extension for the Argentine coach.
"I am, of course, like all supporters of [Marseille], dumbfounded by the sudden decision of Marcelo Bielsa," Labrune said in a statement on the club's website.
"We had just two weeks since found an agreement for the extension of his contract."
Labrune added: "I have tonight difficulty understanding how a simple meeting Wednesday on technical details of the contract with the attorney of the shareholder and my CEO can be the cause of this sudden departure, while all his wishes had been fulfilled.
"The attitude and positivity about Marcelo Bielsa on Thursday during his first press conference of the season [the day after the meeting] are also out of step with the public analysis he has just made."
Labrune announced that assistant coach Franck Passi will take over as interim boss of Marseille, while the president insisted club will have a successful season "despite the departure of Marcelo Bielsa".
In his one season in charge at the Stade Velodrome, Bielsa led Marseille to a fourth-place finish - although just eight wins in Ligue 1 in 2015 cost them a shot at the title.
Marseille finished Saturday's match with 17 shots and 70 per cent possession but Caen prevailed thanks to Andy Delorte's stunning 20-yard drive - one of eight efforts on goal from the visitors.
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