Robinho chose Man City exit after failing to get Mancini guarantee
The News of the World says the Brazilian demanded assurances from the City manager that he would play more in the build-up to the World Cup.
And Robinho made his final decision to leave for Santos on loan after being told he would only start in a third of City's games.
The striker, 25, had been advised by national team boss Dunga that he needed to play regularly to earn a place in Brazil's squad for South Africa.
"I was not playing at Manchester City, right?" said Robinho. "So I was honest with Mancini. I told him I want to play in the World Cup and that I have to play all the games I can until then.
"He told me he already has his team. I replied that I can't play one game and not play the next two, and he understood. Dunga's message to me was very clear - if I don't play regularly for my club side, then I do not go to South Africa.
"I will come back to City in the summer and prove that I'm not a player who sits on the bench.
"This is not just an escape from English football. I have thought about the move seriously and I thought I should have been in the starting team at City."