The move, unprecedented in recent memory, rendered Mr Sahay the shortest-serving DGP of West Bengal.
Sahay, an IPS officer of the 1988 batch, was initially offered the appointment since he was the senior-most officer in the force. But owing to his scheduled superannuation on May 31, well ahead of the formal conclusion of the Lok Sabha elections on June 4, the poll panel considered naming Mukherjee, a 1989 batch officer, as the DGP, a senior official said.
Mukherjee, the Director General of the state Fire & Emergency Services was the second on the list of the three officers recommended by the Bengal government for the post of DGP to the ECI after the poll panel directed the removal of Rajeev Kumar from that position in the run-up to the elections.
On Monday, following an instruction of the ECI to post Rajeev Kumar to a "non-election related assignment", and, as an interim arrangement, post an officer immediately junior to him as DGP, the Bengal government had named Sahay as replacement.
Curiously, while Sahay's appointment notification was made by the state home department in which it referred to the ECI order, the corresponding notification for Mukherjee a day later was made directly by the Commission.
Explaining the sudden change of mind about the appointment of the state's top cop, a senior EC official told PTI that Sahay's was only an "interim appointment".
He did not justify, though, why an appointment was notified by the state government would be toppled in less than 24 hours.
Another official of the poll panel, however, said that Sahay was named the DGP based solely on his seniority and experience.
"But since he is supposed to retire in May even before the final round of polls on June 1, the ECI decided to make this change. If Sahay remained in the post of DGP, he would have to be changed again in the middle of the election process. That's why Mukherjee was given the charge instead," he told PTI.
A former IPS officer in West Bengal cadre who served the state in senior positions, however, told PTI that the Commission retains the power to extend the tenure of an official if he/she was marked for any special assignment, such as the conduct of elections, by the poll body itself.
"I am not convinced that the retirement clause is the only grounds for removing Sahay," the former officer, requesting anonymity, said.
"Sahay could have been easily retained as the DGP of the state till the elections got over. I believe we never had a DGP for such a small duration ever before in this country," he said.
In March 2021, the ECI had suspended Sahay, then serving as director of security, following an incident in which Chief Minister Mamata Banerjee suffered a leg injury in Nandigram while campaigning for the assembly election in the state.
The decision to suspend Sahay was based on reports submitted by the then chief secretary of West Bengal.
Banerjee received injuries on her left foot in Nandigram on March 11, 2021, while greeting her supporters standing on the footboard of her vehicle when her foot got caught between the front door and the seat of the car because of pressure from the crowd.
Incidentally, the poll panel had also removed Rajeev Kumar from active election management-related duties twice before -- during the 2016 assembly polls and the 2019 general elections.
Kumar was on Monday shifted to the Information and Technology department, an assignment not related to the elections.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)