Ruben Amorim warned his players they are not in the final yet – despite a stunning win in Bilbao. United swept aside their opponents – fourth in – with ruthless efficiency to put one foot in the final - back at Bilbao's San Mames Stadium – on May 21.
Casemiro started the rout, with United skipper scoring twice, as United raced to a 3-0 lead at half-time over 10-man Bilbao, before seeing out the game in the second-half. A delighted Amorim singled out goalscorers Casemiro and Fernandes, as well as Harry Maguire after helping guide United's young guns in front of a fervent home crowd.
But the Red Devils boss warned the tie is far from over ahead of next week's return leg at Old Trafford and said United could be on the receiving end of such a defeat when the two sides meet again.
Amorim said: "The result is really good. But we have to understand that we struggled a lot at the start. The goal and red card changed the game, but that's a good thing with a little bit of luck.
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"We kept ourselves in the game and anything can change with the right situation. We had chances to score one more, but this is not done. The same result can happen at Old Trafford and we need to be prepared.
"You can feel even in the beginning players like Manuel Ugarte, with a lot of experience, Patrick Dorgu, were a little bit nervous. It's a semi-final. But having players like Casemiro, and Bruno Fernandes helped a lot."
Maguire echoed his boss and said: "It's one foot in the final, but it's not done. It's a great start, to get a victory here, score three goals and a clean sheet. All the pressure will be on us, everyone will be expecting us to go through. But the tie's not done and we'll be really focused at Old Trafford.

"We need to prepare properly and if we do that, we'll give ourselves a great chance. It was a clinical first-half and a professional second-half. The first 20 minutes were difficult. The crowd was super loud and the intensity was high.
"We looked a little bit nervous and made some errors, but we grew into the game, weathered the storm and were really clinical.”
Maguire is being labelled 'Harrydinho' on social media after his brilliant dribbling in the lead up the opener. Maguire said: "I'm sure there are a few nicknames for me out there."
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