United have accepted an £80 million offer from Real Madrid for the Portugal international, who has been the club's leading scorer for the past two seasons.
Ronaldo's desire to one day play for Madrid was no secret but it was the need for a new challenge that was behind the move, according to Tosic.
"He told us he had won all the awards and he wanted to try something else," the winger said. "I don't know if he had already signed for Real Madrid.
"He is going to again be the best player in the world. Last season he scored 42 goals, this time 27. And they all said it's his worst season because it was not enough. In Madrid he will be the best again.
Tosic, joined United from Partizan Belgrade in January but has so far made only three first-term appearances.
Currently with Serbia Under-21s at the European Championships in Sweden, he is hoping he can help fill the void left by Ronaldo's impending departure.
"Cristiano was the best player in the team and so someone will come in to take his place. I hope I will get the chance," he added. "These few games are very important for me and then when we go on tour I want to have a break through."
Admitting he had a few issues adapting to the English game when he arrived, Tosic sought guidance from fellow Serb Nemanja Vidic. But in spite of the issues he had, the 22-year-old says he would not change things for the world as he is living out his boyhood dream.
He said: "In a few games for the reserves, the players kick me and I was wondering what is going on, I am going home, I cannot play here – then I get through that, and now that is normal for me.
"I have been a United fan since the 1999 European Cup. It was an incredible final.
"Once when I was a kid and started football training I saw a picture of the national team of Serbia and I said to the coach 'I am going to stand there one day and I am going to be a Manchester united player – those are my two wishes'."