Mendes jetted into the UK at the weekend and held crisis talks with his superstar client over his future at Old Trafford, according to The Sun, UK.
The 36-year-old has scored 14 goals in 21 appearances for Manchester United this season.
Manchester United are 22 points behind leaders Manchester City in the league table, having lost six games.
The Sun reports that the Portugal international fears his dream Old Trafford homecoming could end in disaster.
Mendes, according to the report, recently flew over to sit down with Ronaldo to discuss his concerns about United.
Ronaldo is said to be frustrated with the team’s recent display, while he has also become disillusioned with Rangnick’s style of play.
A source told the newspaper, “Cristiano is very concerned about what is happening at United. The team is receiving a lot of criticism and he knows he is regarded as one of the leaders.
“There are quite a lot of problems and Cristiano is feeling the pressure and is very stressed by the situation.
“He desperately wants his United move to be a success, but he is starting to recognise that winning trophies with the current set up could be a real struggle.
“Jorge came to see him and they spent time chatting through how things are going, what they think are the problems, how they might be solved and what the solutions may be. Nothing has been ruled out.”
Rangnick’s appointment was expected to turn things around at the club but unfortunately, nothing has really changed with a series of lacklustre performances that have seen the club fall to seventh place, 8 points behind fourth-placed Arsenal in the league table.
The club must do every they can to win their FA Cup third-round tie against Aston Villa on Monday night to calm things.
Rangnick is also said to have met two big issues which are holding back his efforts to transform the team.
The first is the defence, with the German concerned that his high-pressing style is not suited to the defenders, while another issue that Rangnick and his staff have identified is a lack of physical conditioning