Spanish left-back Alvaro Fernandez spent three years in the Real Madrid youth system from 2017 to 2020 before being snapped up by the Red Devils, who signed him on a free transfer.
Fernandez hit the ground running in the Man United youth setup - proving his worth as an attacking left-back - and he made the Red Devils' bench for their last four games of the 2023-24 Premier League season under Ralf Rangnick.
The 21-year-old subsequently enjoyed a strong loan spell at Preston North End in the 2022-23 season, coming up with six assists in 42 matches for the Championship club, but he did not do enough to get into Erik ten Hag's good books.
Instead, Fernandez has spent time on loan with Spanish side Granada and Portuguese giants Benfica this season, making 16 appearances for the latter and contributing one goal and one assist.
Benfica trigger option to buy Fernandez
Benfica and Man United's loan agreement for Fernandez also included a permanent option, and on Sunday evening, the Red Devils confirmed that the Portuguese outfit had exercised that right.
Fernandez has penned a five-year contract with the Eagles until the summer of 2029, and Man United are expected to receive around €6m (£5.1m) for the sale of the Spain Under-21 international.
"I'm very happy for the trust they've placed in me. I've wanted to keep wearing this jersey ever since I arrived, and I want to say thank you next year by fighting for every possible title with the support of the fans," Fernandez told the official Benfica website.
Meanwhile, Man United's statement also included a well-wish to the defender, saying: "Everyone at United would like to wish Alvaro all the best of luck in the future and will be keeping an eye on his progress."
Fernandez leaves Man United without ever making a competitive appearance for the Red Devils' first team, and he only made one of Ten Hag's matchday squads in August's 3-2 Premier League win over Nottingham Forest.
Which players could depart alongside Fernandez?
While Fernandez is the first player to leave Man United for another club this summer, the Red Devils have already confirmed that Raphael Varane will be departing when his contract expires at the end of next month.
Injury-riddled striker Anthony Martial and out-of-favour defender Brandon Williams are expected to follow suit, and it is unclear whether veteran pair Tom Heaton and Jonny Evans will be offered the chance to stay on, but the former is supposedly keen to extend.
However, Sofyan Amrabat's loan move from Fiorentina should not be made permanent, while fellow midfielders Christian Eriksen, Casemiro and Donny van de Beek are all on the chopping block too.
Further forward, neither Jadon Sancho nor Mason Greenwood have a future at Old Trafford, while the futures of Hannibal Mejbri and Facundo Pellistri - on loan at Sevilla and Granada respectively - are clouded in uncertainty.
Bids will likely be entertained for Antony and Aaron Wan-Bissaka too, but the biggest question undoubtedly surrounds the future of head coach Ten Hag, allegedly set to be sacked in spite of Saturday's FA Cup final victory over Manchester City.