David Moyes's side had been hoping for the FA Cup, and Tuesday's League Cup semi-final first leg at Sunderland, to provide some relief from his team's Premier League travails.

But the 80th-minute dismissal of substitute Fabio da Silva -- shown a straight red for a rash challenge on Jose Canas -- set the stage for Swansea to claim a dramatic first ever win at Old Trafford.

Wayne Routledge's opening goal was quickly cancelled out by Javier Hernandez, but in the last minute, Routledge beat stand-in right-back Darren Fletcher and crossed for Ivorian Bony to head home from six yards.

Swansea's reward will be a trip to either Birmingham City, Bristol Rovers or Crawley Town in round four.

Faced with the prospect of two domestic cup ties in the space of 48 hours, United hardly made the most auspicious of starts, going behind to Routledge's opening goal after just 12 minutes.

It was a goal carved out by the sort of slick, one-touch passing football for which Swansea have become renowned under manager Michael Laudrup.

Bony and Alejandro Pozuelo exchanged passes and the latter caught United's back line square with a perfect pass.

Routledge did the rest, sprinting ahead of Chris Smalling before lobbing Anders Lindegaard in the United goal with a perfectly judged finish.

It was a big test of United's resolve, on the back of a home loss to Tottenham Hotspur in their last outing, and their response was swift, with Hernandez equalising thanks to strong work from Alexander Buttner.

The Dutchman whipped over a dangerous cross from the left and the Mexican forward arrived between markers Chico Flores and Neil Taylor to finish from close range.

United just about deserved to be level, based on the first half, but as has been the case throughout much of the season to date, they were far from their former best.

Hernandez had an early run and shot well blocked by Dwight Tiendalli and following the exchange of goals, Antonio Valencia poked the ball narrowly over after Danny Welbeck's tidy lay-off.

In response, a Jonathan de Guzman free-kick, steered directly at Lindegaard, was one of the few first-half glimpses the visitors had of the United goal.

United responded quickly in kind, with Buttner firing in a direct free-kick that was comfortably saved by Gerhard Tremmel.

Hernandez, with a misdirected header and a mishit volley, spurned a couple of other half-chances before the interval, but the visitors opened the second half in the ascendancy.

Pozuelo's committed run opened a shooting opportunity for Routledge and won a corner, before the Spanish forward made room for himself to take a low shot that skidded just wide of the home goal.

Bony also shot well wide at the end of another Swansea attack as the tie hung in the balance.

Fresh injury concern arrived for Moyes as defender Rio Ferdinand, making his first appearance since the start of December, limped off with a knee injury in the 75th minute after an accidental collision.

By that stage, Moyes had brought on young winger Adnan Januzaj in an attempt to breathe some life into his team's attacking play, with the absences of injured duo Wayne Rooney and Robin van Persie keenly felt.

But the rapid dismissal of Fabio was another blow for the home side, before Bony turned out the lights on their hopes of reaching round four.