Habas has been the coach of the Kolkata team for four years in total. Under him, ATK had won the ISL in 2014 and 2019.
Mumbai are only the second team, after Bengaluru FC in 2018-19, to finish on top of the league and win the final in the same season.
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Now, the Fijian has an opportunity to make up for that missed opportunity, as he stands on the verge of winning the Golden Boot this seaso
With this win, the Mariners have set up a blockbuster title clash against League Shield Winners Mumbai City FC.
Antonio Habas was just minutes away from recording his maiden first-leg playoff win in the League when the second-half substitute struck in the fourth minute of injury time to cancel out David Williams' 34th-minute goal.
ATK Mohun Bagan, who finished second in the league stage, will lock horns with third-placed NorthEast United FC.
Sergio Lobera's side comfortably won after first-half goals from Mourtada Fall (7th minute) and Bartholomew Ogbeche (39th).
As the Kolkata derby marks its 100th year, since the Cooch Behar Cup in 1921, SC East Bengal and ATK Mohun Bagan find themselves in contrasting positions in the ISL table.
After a cagey affair threatened to end in a goalless draw, Krishna's late strike (85th minute) was the difference as Antonio Habas' team grabbed all three points at the Fatorda Stadium.
The winner of the ISL league phase -- with four rounds left to play -- will get an automatic qualification in the AFC Champions League next season.
The group stage draw of the continent's second-tier club competition involving 39 teams -- an increase of three from 2019 -- was held online here.
Gallego had a part to play in Luis Machado's opener (60th) before Roy Krishna (72nd) equalised for the Mariners. Gallego's 81st minute curler settled things for NorthEast.
Both the teams played an exciting and evenly fought match, trading missed opportunities.
Mumbai City are now five points clear at the top of the standings over the second-placed Mariners right at the halfway stage of the league.
ATK Mohun Bagan are two points adrift of Mumbai City with six wins, two draws and one loss. They are also on a five-match unbeaten streak.
ATKMB started off slowly and there were not many clear cut chances from both the teams.
Just once point separated table-toppers Bagan and Mumbai City FC before the start of the latter's match against Kerala Blasters on Saturday.
Against ATKMB, who are yet to concede from open play this season, the likes of Jakub Sylvestr will have to do a lot better if Chennaiyin are to walk away from the match with something.