Mohun Bagan chief coach Sanjoy Sen quit his post after the Kolkata football giants went down 1-2 to Chennai City FC in the I-League, at home.
The southern outfit took a sixth minute lead through Frenchman Jean-Michel Joachim. But they were reduced to 10 players after Pradeep Mohanraj was sent off in the 34th minute for handling the ball inside the box.
Ansumana Kromah scored the equaliser from the resultant penalty in the 36th minute but it was Veniamin Shumeyko who headed home the winner in the 71st minute to hand Mohun Bagan their first loss of the season after six matches.
Chennai City FC celebrate their first goal against Mohun Bagan during the I-League encounter in Kolkata | Salil Bera
The lad from Kyrgyzstan brilliantly rose above three defenders to head home captain Michael Soosairaj's corner kick to silence the Maidan crowd.
The loss also exposed the Sanjoy Sen-coached side's dependence on their star Haitian forward Sony Norde who is out with a knee injury.
The familiar chants of 'go back Sanjoy Sen' echoed around the Maidan with thousands booing the coach.
In the last match, too, Mohun Bagan's opponents Indian Arrows were down to 10 men but they failed to make their numerical advantage count in a one-all stalemate.
Chennai City FC and Mohun Bagan players in action during the I-League encounter in Kolkata | Salil Bera
The result also came on a day when their arch-rivals East Bengal beat Indian Arrows 2-0 in New Delhi to consolidate their lead on the points table while Mohun Bagan's title hopes were fast going up in smoke.
Mohun Bagan now have 10 points from seven matches while Chennai City have seven from seven outings after their second win of the season.
