Fireworks crackled and red smoke filled an expectant Old Trafford following an impressive 3-0 win against 10-man Athletic in northern Spain last week.

But the air of excitement was tinged by the knowledge that Ruben Amorim's United have shown a startling ability to implode during a horrendous Premier League season.

Amorim reverted to the team that started in the first leg, making eight changes following Sunday's 4-3 defeat against Brentford.

Dani Vivian was suspended after his red card last week and coach Ernesto Valverde was also hamstrung by the absence of brothers Nico and Inaki Williams as well as top-scorer Oihan Sancet.

Mikel Jauregizar silenced the home fans and gave the visitors hope on the night with a stunning strike in the 31st minute.

For long periods of the match the visitors looked more dangerous but substitute Mason Mount settled the home supporters' nerves with 18 minutes to go.

The match totally changed complexion as Casemiro, Rasmus Hojlund and Mount again scored.

It means United have kept alive their hopes of eking something out of a terrible season.

Winning Europe's second-tier club trophy crucially guarantees a place in the Champions League, which would boost United's chances of attracting top talent as Amorim attempts a monumental rebuild.

United were heavy favourites to complete the job on home turf but injury-hit Athletic nursed a grievance after defender Vivian was sent off in Spain.

They enjoyed the bulk of the early possession but struggled to fashion clear-cut chances and looked nervous when the visitors attacked.

Athletic midfielder Alex Berenguer had a good early opportunity after the home side failed to clear but his curling shot sailed over the bar as he aimed for the top corner.

The midfielder created another chance after a surging run midway through the half but fired wide of Andre Onana's post from just outside the area.

- United nerves -

But the visitors broke the deadlock in sensational style after half an hour to give themselves hope when Jauregizar picked his spot from outside the box and curled past Onana.

The goal came after some sloppy work from United's defence. Harry Maguire played a ball across the box and Alvaro Djalo smashed a shot against Leny Yoro.

The ball fell to Jauregizar, who could not have hit the ball more sweetly.

United struggled for attacking rhythm but should have been level shortly before half-time.

The stadium held its breath as Alejandro Garnacho was put through by the lively Patrick Dorgu but his attempted dink over the goalkeeper went wide.

United offered little going forward in the early stages of the second half as Athletic made the running, forcing United to hack clear on multiple occasions.

Both teams made a triple change just after the hour mark. Shortly afterwards Unai Gomez headed the ball across the goal and it bounced just wide of the post.

Injury-hit Mount has been a peripheral figure in his two seasons at Old Trafford but produced a fine finish, swivelling and curling the ball into the far corner in the 72nd minute.

Casemiro scored a second with his head from a Bruno Fernandes cross as United's earlier nerves were completely forgotten.

Hojlund poked home in the 85th minute before Mount scored an outrageous second from near the halfway line after Athletic goalkeeper Julen Agirrezabala came out of his box and made a poor clearance.

United will face Tottenham in the final in Bilbao on May 21.