Sergi Roberto surprisingly arrived into the first-team setup as a right-back and established himself, despite being a natural midfielder.
They claim Roberto is non-transferable, but with Mourinho and Conte both trying to secure his services, they could try to start a bidding war with both clubs trying to build their squads to challenge for the Premier League title.
The potential for the Spaniard to become surplus to requirements has not only alerted United to his potential availability before the window closes, but Chelsea are also interested.
Mourinho is also hoping to add to his squad before the end of the month and Roberto would be a long-term replacement for Antonio Valencia if he was to make the move to Old Trafford.
United is not alone in showing interest in the Spaniard, as fellow Premier League side Chelsea has inquired about Sergi Roberto, although the London-based club has not made a formal offer.
The Mundo Deportivo report suggests that Barca are unwilling to sell Roberto and have knocked back United's opening offer but the Express claim the defender has a £36m buyout clause which has now seemingly been met.
Roberto made 40 appearances for the Blaugrana last season but despite being primarily a deep-lying midfielder, he was deployed as right-back last season after the club did not replace Dani Alves.
The 25-year-old is a product of the Barca youth academy and has played 153 times for the Catalan giants.