United have consistently set their sights on the world's biggest names under the stewardship of executive vice-chairman Ed Woodward and have made attempts to resign Cristiano Ronaldo in recent times, while they have also pursued Bale and Neymar in the last year.
On Friday, sources close to Messi refused to rule out of the possibility of a move.
They admitted to Sportsmail that the player is increasingly 'frustrated and hurt' by the media circus in Spain that has surrounded a legal case investigating his tax affairs.
One onlooker outside the court shouted 'Go and play in Panama!' - a reference to the recent Panama Papers leak which saw thousands of documents released relating to off-shore tax havens.
The 28-year-old, one of the world's highest-paid athletes, and his father are accused of using a chain of fake companies in Belize and Uruguay to avoid paying takes on 4.16 million euros (£3.2million) of the footballer's income earned through the sale of his image rights from 2007-09.
'I didn't know anything,' Messi said while testifying in his tax fraud trial. 'I only worried about playing football.'
A verdict is expected next week.
Meanwhile, United are confident of completing the signing of Zlatan Ibrahimovic. The striker is a free agent once his Paris Saint-Germain deal expires in the summer and is currently with Sweden, preparing for Euro 2016.
-Daily Mail
