Watch Manchester United games on delay, via MUTV, on beIN SPORTS | Sign Up to get beIN SPORTS CONNECT for $9.99 per month for three months! | AUS | NZ 

Joshua Zirkzee's opener in the first leg was cancelled out by a Mikel Oyazarabal penalty, and La Real's captain did the damage with another spot-kick to open the scoring early on at Old Trafford.

However, Fernandes soon levelled with a penalty of his own before doubling his tally from 12 yards in the second half.

Jon Aramburu was then dismissed prior to the United captain clinching his treble three minutes from time, before Diogo Dalot wrapped up the scoring, sealing a quarter-final meeting with Lyon.

Ruben Amorim's side had earlier fallen the wrong side of a lengthy VAR check as Matthijs de Ligt was adjudged to have fouled Oyarzabal, who coolly slotted his 14th-minute penalty into the bottom-right corner.

However, United struck back at the other end with a penalty six minutes later after Rasmus Hojlund was felled by Igor Zubeldia, and Fernandes stroked into the bottom-left corner from 12 yards.

United came flying out the blocks after the interval as Zirkzee forced a smart save from Alex Remiro, with Patrick Dorgu winning a penalty on the rebound after colliding with Aritz Elustondo.

Fernandes subsequently sent Remiro the wrong way, rolling into the right-hand corner to send Amorim's side ahead in the 50th minute.

Aramburu was then dismissed for a last-man sliding challenge after Zirkzee's delightful pass to Dorgu, who almost won another penalty following Hamari Traore's lunge, before the VAR intervened to overturn the call.

Fernandes would have been the taker of United's third spot-kick, but he sealed his hat-trick just moments later, racing through onto Alejandro Garnacho's offload before drilling into the bottom-left corner.

Hojlund's selfless work capped a fine outing for the hosts as his driving run resulted in a cutback for Dalot, who hammered into the roof of the net when unmarked near the penalty spot.

Amorim described Fernandes as the "perfect" captain after his hat-trick.

"When we need it, [Fernandes] is always there," United's head coach said. "He can bring the ball forward. He can score goals. He is a perfect captain for our team and we need to help him win titles."

"We know that sometimes he is frustrated. We know he wants to win so badly and when things are not going well, he is changing position and going after the ball. Sometimes he needs to trust his team-mates."

Amorim was clearly impressed with how his players dealt with the physical challenges put up by him and those served by playing in different tournaments.

"It was a complete week. We showed a bit of the future. Today, we had a good performance from our team, the physicality was there," he said.

"In this competition it is completely different - we can be the dominant side in physicality. It was a tough week, but a good week.

"I am also learning how to use players like Casemiro. Without a lot of options we are trying to put our players in the best positions. You can feel they have more freedom because of the one game in Spain [first leg against Real Sociedad] we tried to use the pace of Garnacho on the right.

"He has shown me so many things on the right, maybe better than the left."

Hojlund's failure to find the back of the net, meaning it is now 21 matches in all competitions since his last goal for United, remained a disappointment.

"We play with two 10s. We need one player to push the team and make the runs. He his helping a lot. Today, he did three or four connections that can allow us to change the game," Amorim said. "He just needs to score a goal, he is so anxious."