The 20-year-old will make a €37 million [£32m] move to the Allianz Arena in the summer after coming through the ranks at BVB and will play his final game for his boyhood club in the Champions League final against the Bavarians on Saturday.
"It was like a heart attack. It was one day after [the 3-2 victory over Malaga," he told The Guardian.
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"Michael asked if I wanted to talk and I said: 'No, I have to go'. That evening my wife was waiting because there's a very good German actor, and a good friend, Wotan Wilke Mohring, in a new film in Essen and we were invited to the premiere. But I walked in and told her: 'No chance. I cannot speak. It's not possible to take me out tonight'.
"Bayern told Mario: 'It's now or never'. I told him they will come next year. They will come in two years, and then three years. But he is 20 and he thought: 'I must go'."
He added: "I know how difficult it will be to find a player to replace Gotze but, next year, we will play differently. It just takes time."