According to The Athletic, the 37-year-old forward has also been arguing with manager Erik ten Hag over the team's high-pressing approach.
The Red Devils are coming off the back of a humiliating defeat against Brentford at the weekend, where they found themselves 4-0 down after just 35 minutes. Cristiano Ronaldo played the full 90 minutes, but was ineffective throughout the clash and is reportedly keen to leave the club in search of Champions League football.
There were concerns before the start of the season that the veteran attacker would have a problem when trying to fit into Ten Hag's system. The former Ajax boss demands high-intensity from his players as well as tactical cohesion and pressing from the front.
The Dutch tactician's side were torn apart at the weekend, with the latest reports surrounding the Portuguese superstar surely not helping. The Athletic report also claims that Ten Hag is now more open to the five-time Ballon d'Or winner leaving the club this summer, but they have still not received any offers.
Manchester United target Leicester City superstar to replace Cristiano Ronaldo - Reports
The Athletic article also claims that United are now targeting Leicester City legend Jamie Vardy during the window. Vardy, 35, has less than 12 months left on his current deal and his high-intensity and outstanding pressing is seen as an ideal option for the 20-time champions of England.
Vardy is considered to be one of the greatest Premier League forwards, having scored 133 top-flight goals, which puts him 14th on the all-time list. Although the former England forward is a key part of Leicester's team, the Foxes are currently going through a tough spell.
They are yet to sign a single outfield player this season and their club captain and number one goalkeeper for over a decade, Kasper Schmeichel, left for Ligue 1 side Nice just days before the start of the campaign. Several of their key players, including Youri Tielemans, James Maddison and Wesley Fofana, have been linked with moves away from the Midlands club.
Manchester United will certainly need a new striker this summer, especially if Cristiano Ronaldo does end up leaving the club. Edinson Cavani has already departed Old Trafford this summer, while Anthony Martial, who enjoyed an excellent pre-season, has picked up an injury.