It comes after Tottenham pipped them to the signing of Sergio Reguilon, who has joined Spurs from Real Madrid.
They have instead set their sights on 27-year-old Telles, reports RMC, the player has agreed personal terms with United.
A transfer fee is yet to hashed out between the two clubs as PSG also remain interested in the defender.
United pulled out of the race to sign Reguilon, 23, due to Madrid's insistence on inserting a buy-back clause into the deal.
And they turned their attention to Brazilian Telles, who is in the last year of his deal at Porto.
The Portuguese club risk seeing him leave for free at the end of the season if they don't cash in now.