As the half wore on the Reds kept up the pressure but could find no way through a stubborn Athletic defence. The home side lost Llorente five minutes before the break to injury – the striker had been a doubt before the game – with Gaizka Toquero replacing him.
Athletic Club showed no signs of letting up after the restart, with de Marcos blasting a shot just wide. Right-back Andoni Iraola almost added a superb second soon after when, after being found on the edge of the area, he tricked his way past a cluster of Reds before clipping his shot just past De Gea’s left-hand post.
He then went close to turning provider in Athletic’s next attack when he burst down the right and crossed to the unmarked Toquero, but the substitute spurned the opportunity and blasted over.
The Reds were struggling to create any chances at the other end and, with another vital Barclays Premier League clash looming at Wolves on Sunday, Sir Alex opted to withdraw Rio Ferdinand and Michael Carrick. Chris Smalling and birthday boy Paul Pogba entered the fray.
No sooner had the substitutes come on, United went further behind as Athletic Club sealed their spot in the Europa League quarter-finals on 66 minutes. Iraola went on another marauding run down the right and although his cross was half-cleared by Smalling it fell to the feet of de Marcos, who brought the ball under control before blasting low past De Gea via a slight deflection off Jonny Evans. Cue more jubilant celebrations around the Estadio San Mames.
It nearly got worse for Sir Alex’s men when Susaeta screwed a shot just wide and Toquero headed over the bar. Instead, Rooney reduced the arrears with a consolation goal – a lovely curling effort from 25 yards that was sportingly applauded for its quality by the home fans.