Juan Ferrando started the league as FC Goa's chief coach and on Monday officially took over charge of struggling Kolkata giants ATK Mohun Bagan.
The announcement comes just a day before the struggling Mariners take on North East United FC in their seventh match of the season.
"I will give my 110 per cent from day one so all fans can enjoy the best brand of football and hopefully we can celebrate together soon. I will consistently work towards helping our team give their best," Ferrando said in a statement.
The 40-year-old invoked his release clause to part ways with FC Goa in the middle of the season and their president and co-owner Akshay Tandon expressed his surprise and also accused ATK Mohun Bagan of poaching on Twitter on Sunday.
"With disappointment I'd like to confirm that @JuanFerrandoF has triggered his release clause, obliging us to release him of his duties so that he can join @atkmohunbaganfc."
Tandon said that Ferrando left no room for discussion.
"As long the money gets deposited in our accounts, we do not have a choice in his decision. This has come as a surprise to us, @JuanFerrandoF informed us of his decision this morning. He left no room for a discussion or a negotiation," Tandon tweeted.
However, Tandon stated that poaching coaches in this day and world is a fair game.
"Poaching our coaches, staff and players is fair game but I hope better sense and sportsmanship prevail in the future. We would have at least liked the opportunity to inform our squad ourselves. To our fans, I'd like say, 'Do not worry'. This will in fact make us stronger.
"It may not seem like that now. I'd like to ask your love and belief in your club. We are @FCGoaOfficial. We have a championship squad and there is already a full strength of coaches and staff to fill in the gaps in the short term," Tandon wrote.
ATK Mohun Bagan play NorthEast United FC on Tuesday and Ferrando is likely to be in the dug-out.
Ferrando was appointed FC Goa's coach last season and under him the Gaurs made the last-four, going down to eventual champions Mumbai City FC on penalties.
He also created history for FCG which became the first Indian club to earn a point in the AFC Champions League in the group stages earlier this year.
He also coached FC Goa to the Durand Cup title in Kolkata in October, defeating Mohammedan Sporting 1-0.
This season, FC Goa have slumped to eighth in the table, having lost their opening three games -- to Mumbai City, Jamshedpur FC and NorthEast United FC.
However, Ferrando appeared to have reversed their fortunes with two consecutive wins, against SC East Bengal and Bengaluru FC, before a thrilling draw against Hyderabad FC.
Speaking on Ferrando's departure, Director of Football, Ravi Puskur said,"We are very disappointed to lose Juan. His decision to leave the club was rather unexpected and came as a surprise, particularly at this juncture of the season, where we are in the middle of the season."
Pushkar said that he had kept the club in dark.
"We are particularly disappointed that we were kept in the dark until yesterday morning where he communicated his desire to leave the club. However, once he stated his stance and the release clause was activated, we were left with little alternative. I wish Juan the best for his future."
Assistant Coach Clifford Miranda will take interim charge of the First Team.
