Robinho joined Santos this week on-loan to the end of the season.
Mancini said: "I spoke with him and said if you want to stay here I will be happy. But if you want to go to Brazil, you can go.
"He said 'Okay I will go back to Brazil for six months because the World Cup is coming'.
"But after the World Cup I think he can make it here. He will come back here because for me, he is a fantastic player and he has a good talent.
"He can go back to Brazil, stay with his family and play in every game for Santos. That is important to him and I understand that."