Fernandez completed a permanent move to Benfica in the summer transfer window after he spent the second half of last season on loan at the Portuguese club.
Man United agreed to sell Fernandez in a deal worth an initial €6m (£5m), with the Red Devils retaining a two-year buy-back option.
Fernandez had previously spent time out on loan at Preston North End and Granada after he failed to break into the Man United first-team.
In fact, the Spain Under-21 international left the Premier League club without making a single appearance for the senior side.
Fernandez opens up on Man United return rumours
While he may have struggled at Man United, Fernandez has made a positive impressive since completing a permanent switch to Benfica.
The left-back has started nine of his 11 competitive appearances this season, including a third consecutive Champions League start in Wednesday's 3-1 defeat to Feyenoord.
Speaking after the game, Fernandez was asked about a potential Old Trafford return amid speculation linking him with his former club.
Fernandez told Record: "I only think about Benfica. I'm only focused here. I'll try to keep up the same level, try to help the team as much as possible. I'm just another player on the team."
"Not scoring, that's really important to win. We had everything well planned. We weren't surprised by Feyenoord. We knew how they played, quick counter-attacks, but like I said, let's think about the future, the next game, which is on Sunday."
How likely is a Man United return?
With the buy-back clause said to be around £16m, Man United are in a strong position should they want to re-sign their former defender.
However, Ten Hag ultimately deemed Fernandez surplus to requirements in the summer, and it seems unlikely that his position would have drastically changed in such a short period.
But with the Dutchman battling to save his job, there is a possibility that a new Man United manager could look to bring Fernandez back to Old Trafford.
The left-back position is certainly a problem area for Man United due to the injury issues which have plagued Luke Shaw and Tyrell Malacia.