According to Manu Sainz of AS, Ronaldo feels "betrayed" that he has not been offered the improved contract he was promised by club president Florentino Perez after Real won their second successive UEFA Champions League back in June.
Sources close to the Portuguese said: "Cristiano is in the doldrums, as he feels that he has been tricked."
Sainz added Ronaldo intends to leave Real in the summer, with Los Blancos said to be eyeing up Paris Saint-Germain duo Kylian Mbappe and Neymar as potential replacements for the 32-year-old.
Ronaldo's contract, signed back in Nov. 2016, runs to 2021.
The former Sporting CP star was instrumental as Real won a Champions League and La Liga double last season, but his form has dipped drastically this term.
Ronaldo has netted just four times in 14 Liga appearances in 2017-18, and Real have also endured mixed results, seeing them slip 19 points back from table-topping Barcelona.
Given just how prolific Ronaldo has been for Real since joining the club from United in 2009 for a then-world-record £80 million fee, his woes in front of goal this term have been remarkable.
The Ballon d'Or holder has continued to dazzle in the Champions League, however, netting nine goals in six appearances.
Therefore, his brief run of bad form in the Spanish top flight should not be enough to leave Real happy to see him go, and it seems unlikely they would let him return to United in the summer without resistance.
The latest reports are far from the first time the forward has been linked with his former club, as rumours of Ronaldo's unhappiness and longing for a return to Old Trafford have been commonplace over the past few seasons, per the Guardian's Sid Lowe:
Despite his recent barren run and the fact he will be 33 in February, Ronaldo would likely be welcomed back with open arms at United, especially by Red Devils fans.
He came of age at Old Trafford between 2003 and 2009, winning three Premier League titles and the first of his four Champions Leagues.