Marcelo Bielsa, meanwhile, made just one change to his Athletic side that left Old Trafford victorious last week: defender Fernando Amorebieta, suspended for the first leg, took the place of Mikel San Jose in the heart of defence.
Despite the fact it was United in need of goals - two of them, at least - it was Athletic, buoyed by a raucous home support, who made the early running with some quick, incisive passing and excellent movement in attack. The impressive Markel Susaeta, who had been one of the star performers seven days earlier, saw his deflected free-kick hit the side-netting on three minutes before another effort hit a United player and dropped kindly into the hands of David De Gea.
United kept Athletic on their toes on the break and goalkeeper Gorka Iraizoz had to be alert after nine minutes when neat interplay between Rooney and Cleverley led to a shot from Young that was bravely blocked by Javi Martinez.
The Reds breathed a sigh of relief soon afterwards when Llorente slid a pass through to the tricky Iker Muniain in the area and his low drive smacked against the post. De Marcos then fired the follow-up over the bar when it looked easier to score. You always felt that should Athletic net the opening goal the tie would be all but over. Unfortunately, that was exactly how things panned out.