The Portuguese global has been heavily linked with a move to the Bundesliga side as he has admitted he wants to leave Spain following his tax evasion charge.
Ronaldo joined United in 2003 aged 18 and went on to score 118 goals in 292 appearances for the Red Devils in six years.
Why does Ronaldo want to leave?
Amid uncertainty over Ronaldo's future, Manchester United and Paris Saint-Germain are said to be exploring the possibility of landing his signature. We will work hard to ensure that Real Madrid are named the best club of this new century.
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And according to Spanish sports daily Marca, Ronaldo has told his teammates: "I'm leaving Real".
If Ronaldo does leave Real, United will still gain even if he isn't back tearing it up at Old Trafford next season.
Ballack, who exited Bayern under something of a cloud when he left for Chelsea on a free transfer in 2006, expects Ronaldo to remain at Zinedine Zidane's La Liga and European champion.
Ronaldo has helped Real win the Champions League thrice, scoring twice in last month's final as they became the first club to retain the title, and he is also the competition's record goal scorer with 105 strikes.
For all the words written and spoken about the four-time Champions League victor, the only people who really know are Ronaldo himself and long-time adviser Jorge Mendes.