Mumbai City FC went into the lead with Matias Defederico finding the target in the 27th minute but the home side were jolted when midfielder Pronay Halder was sent after being shown two yellow cards.
The numerical advantage worked in Kolkata's favour and they forced the equaliser in the 82nd minute through Javier Lara to stay unbeaten in the league.
Atletico de Kolkata now have five points from three matches, while Mumbai City, also unbeaten, are at the top with seven points.
Forlan's absence paved the way for Boithang Haokip to start for the first time and the talented youngster was the one who had the best looks at the rival goal.
More From This Section
Haokip's first chance was as early as the ninth minute when Defederico picked him up with a wonderful through ball but Atletico de Kolkata goalkeeper Debjit Majumder left his charge to avert the danger even though there seemed to be a possible contact.
Mumbai City got their deserving lead, thanks to some brilliant work from Defederico in the 27th minute. The Argentinean showed remarkable skill to control the ball with his first touch and then did even better to leave two defenders behind and slot the ball past a diving Atletico de Kolkata goalkeeper.
Borja Fernandez and Lara were doing all the orchestrating and their best chance to equalize came in the 65th minute.
Lara's cross from the right fell unexpectedly for Sameehg Doutie inside a crowded Mumbai goalmouth but Doutie only managed to tap the ball wide from handshaking distance.
The match swung in the visitors's favour in the 72nd minute when Mumbai midfielder Pronay Halder was sent off for a second bookable offence.
Atletico de Kolkata capitalised on the numerical advantage and got their equaliser through Lara. The Spanish midfielder was class personified as he got away from Jackichand Singh on top of the box and let got a fierce shot that sailed into the top corner of the nets.