United successfully fought off Madrid's attempts to sign Ronaldo last summer, but expect a fresh approach in the coming months.
But Sir Alex Ferguson, the United manager, says Madrid's heavy recent decent against Liverpool and Barcelona have made the Bernabeu seem a less tempting option for Ronaldo.
"Our players were telling him that if he goes to the Bernabeu he'll have to play centre-half," said Ferguson after United wrapped up their third straight title at the weekend.
David Gill, United's chief executive, also remains confident that Ronaldo will stay.
"He's been fantastic both on and off the pitch throughout, he's supported us in what we want to do commercially and he's been a model professional," said Gill.
"He is not for sale. He's a great player and he's part of our club."
Should Ronaldo stay, then United's quest for a record 19th title would certainly be more straightforward. And Ferguson admits plans for next season are already under way.
"I am already thinking about next year," he said. "You have to do it here. There is nothing else for it. You just drive on."