The 20-year-old is currently on loan at Granada, making 14 appearances for the La Liga outfit this season, but his time at the club is set to come to an end this month.
There has been a host of speculation surrounding his future over the last week, with Benfica believed to have stepped up their interest in the left-back.
According to transfer expert Fabrizio Romano, Fernandez will travel to Portugal on Monday to undergo a medical with the Eagles ahead of a move.
Romano claims that the defender will join Benfica on loan for the remainder of the campaign, but there will be an option to make the move permanent this summer.
Fernandez spent the 2022-23 campaign away from his current side, representing Preston North End in the Championship, featuring on 42 occasions in all competitions.
The defender, who arrived at Man United from Real Madrid in 2020, has been a regular for the Red Devils at youth and Under-23s level, but he is yet to make a first-team outing.
Fernandez had seemingly been in line to stay at Man United over the summer due to injuries for both Tyrell Malacia and Luke Shaw, but Erik ten Hag's side instead sent the youngster on loan to Granada and brought in Sergio Reguilon on a short-term basis from Tottenham Hotspur.
Reguilon has since returned to Spurs, while Shaw is back to fitness, but Malacia remains unavailable, with the Netherlands international still recovering from a knee injury.
Romano has not mentioned the size of the buy-out fee, but Record recently reported that Benfica would be prepared to pay around €7m (£6m) for the full-back.
There is no immediate route into the first team at Man United for Fernandez, with Shaw the first-choice left-back, while Malacia impressed in his debut season after arriving from Dutch football.
Man United have already sanctioned three loan exits this month, with Donny van de Beek moving to Eintracht Frankfurt, Jadon Sancho to Borussia Dortmund and Joe Hugill to Burton Albion.
The Red Devils are open to incomings, although the club are working to a strict budget due to the rules surrounding Financial Fair Play, while Sir Jim Ratcliffe's arrival as a minority shareholder has not yet been officially ratified by the Premier League.