Wilfried Bony snatched a dramatic last-minute winner as Swansea City heaped fresh misery on Manchester United with a shock 2-1 success in the FA Cup third round on Sunday.
Eleven points off the pace in the Premier League, where they have already lost six games, United crashed out at the first hurdle in the FA Cup to give beleaguered manager David Moyes even more problems.
To compound matters, United's injury list swelled further after Rio Ferdinand had to go off with a knee injury, while Fabio da Silva was sent off shortly after coming on to replace him.
"We were a bit unlucky to lose, but we probably didn't do enough to win it in the end," said Moyes, whose side visit Sunderland in the League Cup semi-finals on Tuesday.
"We made some opportunities, (but) not enough. When we went down to 10 men, we just needed to see it out and at least take a replay if that was what had to come about, but we didn't quite get that far."
In a game preceded by a minute's applause for late Portugal great Eusebio, who died on Sunday, Swansea took a 12th-minute lead through Wayne Routledge, who cantered onto a pass from Alejandro Pozuelo to lob Anders Lindegaard.
The hosts equalised four minutes later, Javier Hernandez volleying in a cross from Alexander Buttner, but they could not find a second goal.
Ferdinand had to hobble off due to injury late on and his replacement, Fabio, was sent off for a high challenge on Jose Canas shortly after coming on.
Worse was to follow at the death, with Routledge lifting a cross into the box from the left and Bony powering home a near-post header to add a new chapter of disappointment to the tale of United's unravelling season.
Liverpool will visit either Bournemouth or Burton Albion in round four after seeing off Oldham, who stunned Brendan Rodgers's side 3-2 at Boundary Park in the fourth round last season.
Iago Aspas, a close-season signing from Celta Vigo, broke the deadlock in the 54th minute by dispatching a cross from Raheem Sterling to claim his first goal for the club.
Liverpool did not secure victory until Oldham's James Tarkowski put through his own goal with eight minutes to play.