Star-studded Mohun Bagan started the new season on the wrong note, going down 0-1 to Kolkata Port Trust in a Kolkata premier division league match on Monday.
After a barren first half, KoPT struck the winner through Sujit Dey two minutes before the final whistle to score an upset.
Intercepting a pass from Bagan defender Dipak Mandal, Dey set out on a solo, cut into the box and dribbled past Bagan custodian Subroto Paul before putting the ball home.
Bagan put up an insipid performance, looking incohesive and rugged.
The highlight of their display was a spectacular bicycle kick by star striker Bhaichung Bhutia, but the ball sailed over.