Paris St Germain, Bayern Munich and Roma, who are led by the star’s former Real Madrid boss Jose Mourinho, have expressed an interest in the striker.
SunSport has told how Ronaldo, 37, had hoped to end his career at United after rejoining the club earlier this season.
But his agent Jorge Mendes held talks with new Old Trafford supremo Richard Arnold over concerns about his team’s poor form.
They were knocked out of the FA Cup by Middlesbrough and are in a real fight for fourth place in the Premier League after disappointing draws with Burnley and Southampton.
The ex-Real Madrid man is also struggling and will be looking to end his longest goal drought in more than a decade when United host Brighton this evening.
Ronaldo is understood to only wish to stay if United qualify for next season's Champions League.