Villa manager Paul Lambert described the FA Cup as a "distraction" in the build-up to the tie and insisted most of his fellow top-flight managers would rather do without the competition.

That scathing assessment of the world's most famous domestic cup competition came back to haunt Lambert as United made a mockery of the 51 places separating the teams in the league structure.

Nigel Clough's side are languishing in 18th place in the third tier, but they never looked out of their depth at Villa Park and took the lead through Jamie Murphy in the first half.

Nicklas Helenius looked to have spared Villa's blushes when he equalised with 15 minutes left.

But Flynn's superb strike in the 80th minute gave United their first win at Villa since January 1966 and condemned the hosts to a humiliating exit.

Lambert insisted he hadn't meant to disrespect the Cup and made it clear he was disappointed to have gone out in such tame fashion.

"I never demeaned the competition one bit. I respect its history," he said.

"The context of the whole interview was missed. I was not decrying the competition at all.

"I am bitterly disappointed (with the result), especially after a really good win on Wednesday."

Clough refused to condemn Lambert and instead focused on his own players' achievement.

"I don't think he (Lambert) was disrespecting the FA Cup - he was just stating very honestly what the situation is like in the Premier League at the moment," Clough said.

"For us and our 6,000 fans who came today, I think you saw how much it meant. It's still a special competition."

Despite his claims that he could do without the Cup, Lambert made just three changes to the line-up that won at Sunderland in the Premier League on New Year's Day.

That just made Villa's defeat even more dispiriting and Lambert might have feared it wasn't going to be their day when defender Nathan Baker had to withdraw after suffering an injury in the warm-up.

The Blades took the lead in the 20th minute when Murphy made the most of some hesitant Villa defending to cut in from the left flank and drill a hard low shot which appeared to take a deflection off Andreas Weimann on its way past goalkeeper Jed Steer.

Christian Benteke missed two glorious chances to equalise for Villa in the final minute of the first half, twice heading wide after misjudging crosses from Marc Albrighton.

Villa struggled to muster any attacking threat in the second half until the 70th minute when Albrighton set up Benteke who toe-poked his shot straight at United goalkeeper George Long.

Lambert's team finally grabbed a scarcely deserved equaliser in the 75th minute when Weimann's shot took a big deflection and found young Danish midfielder Helenius, who fired home for his first ever Villa goal.

But Flynn restored United's lead in the 80th minute, cutting across the edge of the area before driving a fine left-foot shot past Steer to send 6,000 visiting fans wild and leave Villa to face a chorus of boos from their own supporters.