The final will be held at Estadio San Mames, and while the Red Devils showed intent to ensure they would be there rather than Athletic, Alejandro Garnacho’s early goal was ruled out after the Argentine strayed narrowly offside.
The hosts swiftly pushed forward and came agonisingly close to taking the lead, with Alex Berenguer seeing his shot palmed wide before delivering a cross that Iñaki Williams headed marginally over.
Just moments after being bailed out by Casemiro, Victor Lindelof made two vital blocks to stop Williams’ cross and Berenguer’s shot.
There was no stopping the Red Devils at the other end, as their temporary unorthodox winger Harry Maguire twisted Mikel Jauregizar inside out and delivered a cross that Manuel Ugarte got his head on, diverting it to the back post for Casemiro to head home.
Things got even worse for the hosts when Dani Vivian pulled back Rasmus Hojlund, earning a red card and conceding a penalty that Bruno Fernandes converted with ice-cold composure.
Ernesto Valverde responded with a double change in the 42nd minute, but despair filled the San Mames again three minutes later due to another cool finish from Fernandes.
Noussair Mazraoui crashed a shot against the crossbar in the first half’s final seconds, and the Red Devils were willing to be patient after the restart while Athletic and their supporters made their frustrations known.
A speculative strike from Ugarte shortly after the hour mark was the half’s first attempt, and Casemiro had a shot from range tipped over by Julen Agirrezabala shortly after, before the Brazilian headed against the post from the ensuing corner.
Agirrezabala denied Fernandes and Garnacho late on, but United weren’t troubled on their way to convincingly earning a first win inside 90 minutes from their last eight matches across all competitions, taking a massive step towards the final in the process.
They also inflicted just the second defeat Athletic have suffered in their last 11 matches, with the other coming against Real Madrid, and the Spanish side now have an uphill task to be part of the party they will host.
Flashscore Man of the Match: Bruno Fernandes (Manchester United)