The Daily Star says Ronaldo has given United fresh hope they can tempt him back to Old Trafford by dismissing talk he is signing a new contract at Real Madrid.
United have been working on a deal to sign the brilliant winger in a £60m switch this summer.
They are hopeful of piecing together a £250,000-a-week offer along with club sponsors Chevrolet and Nike to make the 28-year-old their biggest ever signing.
The Portuguese star tweeted: "All the news about my renewal with Real Madrid is false."
Such a step is highly unusual for Ronaldo, who in the past has maintained his silence when speculation over his future has been rife.
And it will provide encouragement to United officials that they can still pull off the transfer deal of the summer to start the David Moyes era with a bang.