(CNN) -- It looked like Manchester United was in the midst of a familiar charge after it beat Norwich at the end of December for a sixth straight win.
But the momentum was snapped by Tottenham to begin 2014 and the Red Devils suffered their second consecutive defeat at Old Trafford when they were humbled by modest Swansea 2-1 Sunday in the third round of the FA Cup.
A late red card brandished to Fabio proved costly.
Swansea had never won at the Theatre of Dreams in 10 previous visits and only returned to the top flight of English football three years ago following a near 30-year absence.
United manager David Moyes didn't exactly field the reserves, either, although Wayne Rooney wasn't in the squad as he continues to nurse a groin injury and Robin van Persie remained sidelined.
"When you lose games at any club, it's difficult," Moyes told BT Sport. "We've got a big game on Tuesday (in the League Cup semifinals) and we'll get the team ready for that.
"We were a bit unlucky to lose. We had some opportunities, not enough, and we were down to 10 men."
Wayne Routledge opened the scoring for Swansea -- which visits Old Trafford in the Premier League next Saturday -- in the 12th minute but Javier Hernandez leveled four minutes later.
Instead of going into another gear as it had done so often in the past under former boss Alex Ferguson, United allowed the Welsh side to stay in the game.
"We played quite well at times, got near the edge of the box quite regularly but didn't make an opportunity to score the goal," said Moyes. "I thought for long periods we had good control of the game."
Fabio was dismissed for an 80th-minute challenge on Jose Canas and 10 minutes later, surging striker Wilfried Bony headed home Routledge's cross to hand United a fourth loss in its last six outings at Old Trafford.
Bony scored twice against United's city rival, Manchester City, on Wednesday.
"We were coming here to not lose, and this is perfect for us," Bony told BT Sport.
"I think the key to the game was the red card. We had to do our best to get one more chance."
Chelsea and Liverpool earlier survived upset bids from Derby and Oldham, respectively, in football's oldest knockout competition, both winning 2-0.
Sunderland, last in the Premier League, beat third-tier Carlisle United 3-1 but West Ham's struggles continued as Nottingham Forest of the Championship crushed the Hammers 5-0.
The fourth round draw produced only one all-Premier League tie, Stoke at Chelsea.