The Spaniard joined the Portuguese giants on a six-month loan deal on Tuesday.
Benfica have an obligation to buy the 20-year-old if he plays a certain number of matches – believed to be 50% – while they also have an option to buy if that figure is not triggered.
Fernandez has spent the season on loan at Granada, but he made just five appearances in La Liga.
United agreed to send Fernandez on loan, despite long-term injuries to Tyrell Malacia and Luke Shaw.
That led Fernandez to claim Erik ten Hag ‘did not count on me’.
But United clearly feel Fernandez could eventually make it at the club, after inserting a buyback clause in his deal to join Benfica.
It’s not known how much United would have to pay to re-sign Fernandez but they’ve also included a significant sell-on clause.
Sergio Reguillon joined United on loan in August but the club cut short his loan spell, with the Spaniard returning to Tottenham.
Shaw is expected back for the trip to Wolves in a fortnight’s time, while Malacia is also closing on a return.
For more stories like this, check our sport page.
Follow Metro Sport for the latest news on
Facebook, Twitter and Instagram.