A plane paid for by a United fan group circled above Villarreal's EL Madrigal Stadium trailing a banner that read "COME HOME RONALDO - UNITED REEL."
Ronaldo left United for Madrid in 2009 on a then-record transfer of 93 million euros.
He has since helped Madrid win its 10th European Cup and one Spanish league title.
The Portugal star won the Ballon d'Or award for the world's best player last year.
He is under contract with Madrid until 2018.