Manchester United
Manchester United's Javier Hernandez, Michael Carrick and Ryan Giggs prepare to kick off after Athletic Bilbao's Oscar de Marcos scored in Manchester.

Sir Alex Ferguson has denounced the refereeing decision that preceded Athletic Bilbao's controversial winning goal and threatens to end Manchester United's Europa League campaign. But the Scot admitted his team's defending was below par in the 3-2 defeat at Old Trafford on Thursday night.

Defeat in the last-16 first-leg tie in Manchester leaves United's hopes of progressing further in this competition hanging by a thread, with Bilbao taking three important away goals back to Spain ahead of the return leg on 15 March.

Ferguson still believes his Manchester United side can reach the Europa League quarter-finals but he was left to bemoan a series of defensive mishaps and a contentious refereeing call that sullied a promising start by the home side.

Wayne Rooney had given Manchester United the lead after 22 minutes, firing in from close range before Fernando Llorente's header drew the visitors level.

The home side were left rocking after the break as goals from Oscar de Marcos and Iker Muniain gave Bilbao a 3-1 lead before a late Rooney penalty salvaged Manchester United's hopes.

But Ferguson questioned referee Florian Meyer's decision to award a free kick to the Spanish visitors in the build-up to their third goal because Patrice Evra lost his boot.

"The referee said you can't play without a boot but the real rule is you can but you have to go off the field so it seems a bit bizarre," Ferguson is quoted in the Telegraph.

"Half the team were still arguing with the referee when they scored the third. It was unfortunate for us but Bilbao were better."

Bilbao manager Marcelo Bielsa was sympathetic to Manchester United, saying: "Both parties are innocent. I don't know if there is a rule which says you can't play without a boot. Certainly Evra wasn't aware of that. It was one of those unfortunate things."

But Ferguson acknowledged that his side's defending was the main cause of yet another home defeat in Europe.

"We were well beaten," Ferguson told the official Manchester United Web site. "Bilbao were the better team, they were very aggressive in terms of their attacking and pressing the ball. Their system caused us problems. All teams do that away from home - they overload the midfield which can cause you problems.

"Without David De Gea, we could have conceded four or five goals. He was superb. He made maybe four or five terrific saves in the game."