After a promising summer with United's senior squad, the 20-year-old spent the first half of the campaign at La Liga outfit Granada CF, making 13 appearances in all competitions.
He was recalled earlier this month and will now see out 2023/24 with the Primeira Liga champions, who are bidding to win a record-extending 39th top-flight crown.
Benfica are currently a point behind city rivals Sporting Club at the summit of the Portuguese table and have a familiar face in their ranks, in one-time United winger Angel Di Maria.
They also have Europa League participation to look forward to, having been eliminated from their Champions League group in December.