The prolific Portuguese has made clear to Erik ten Hag that his intention is to join a club playing Champions League football this season, though the United manager played down suggestions the 37-year-old forward could leave Old Trafford.
But with United waiting on international clearance to register Brazil forward Antony — whose arrival from Ajax could end up costing United as much as €100 million — Ronaldo's second stint at United could be coming to an abrupt end.
Ronaldo scored 24 goals last season as United finished sixth in the Premier League but has yet to register this campaign and has started the last two games — both United wins — on the bench.
Despite that healthy goal return, Ronaldo's return to the club after stints at Real Madrid and Juventus has drawn criticism that the player upsets the balance of the team due to his perceived lack of work ethic off the ball.
Asked about Ronaldo's future ahead of Thursday's clash against Leicester City, Ten Hag said: “We are happy with him, he is happy to be here and we want to make the season a success together.”
Napoli and Sporting - the club where Ronaldo began his career as a precocious 16-year-old winger - have been linked with international football's record scorer, though Ronaldo's wages are likely to prove a stumbling block.
United have also been linked with former striker Memphis Depay, who has been encouraged by Barcelona to seek a move away from Camp Nou with Robert Lewandowski firmly established as first-choice.
Another of Barca's Dutch contingent, midfielder Frenkie de Jong, has also been strongly linked with a move to Old Trafford, though the Netherlands international's preference is to remain in Spain.
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Chelsea, too, seem to be in no mood to put the chequebook away after finally getting their hands on £70 million Leicester defender Wesley Fofana on Wednesday.
The Blues have been credited with interest in Barcelona’s former Arsenal forward Pierre-Emerick Aubameyang, Crystal Palace winger Wilfried Zaha, AC Milan’s Rafael Leao. and Arsen Zakharyan of Dynamo Moscow amid claims they got nowhere with Paris Saint-Germain’s Neymar.
However, Everton’s Anthony Gordon appears to remain out of their grasp.
Liverpool manager Jurgen Klopp has admitted he might need another midfielder and that was before Jordan Henderson suffered an injury during the win over Newcastle United on Wednesday. Aston Villa’s Douglas Luiz has been touted as a possible target for the Reds.
One midfielder who could be up for grabs during the final hours of the window is Youri Tielemans, with reports suggesting Leicester City are open to offers of £25 million for a player who is in the final year of his contract.