Swansea - Manchester United manager Louis van Gaal blamed a five-minute lapse in concentration for his side's 2-1 defeat at Swansea City in the Premier League on Sunday.
Protecting an unbeaten record, United took a 48th-minute lead through Juan Mata at the Liberty Stadium, only for goals in quick succession from Andre Ayew and Bafetimbi Gomis to give Swansea a third successive win over their opponents.
Swansea's goals arrived shortly after Garry Monk had switched his formation to a 4-4-2 with a midfield diamond, moving Ayew up front alongside Gomis, and Van Gaal said his players had not reacted to it quickly enough.
"The opponent changed their shape. They had started with a 4-2-3-1 and after 1-0, they changed their shape and then we couldn't cope with that," said the Dutchman, who was presiding over his 50th United game.
"That's not good because we were only drawing for five minutes and we have lost what we had in five minutes.
"And that was a pity because I think we controlled the game and we played very good."
Mata concurred that his side had paid the price for not readjusting after Swansea altered their approach.
"It is a big disappointment. We scored first and then in five minutes we lost the game," the Spanish midfielder told Sky Sports.
"They changed the shape, we didn't react and we haven't learnt a lesson from last season."
The result left Swansea a point above United in fourth place in the Premier League table, four points shy of early leaders Manchester City.
After equalising with a 61st-minute header from Gylfi Sigurdsson's cross, Ayew set up Gomis's winner with a sublime pass from wide on the right and the Ghana international said the assist had given him more pleasure than the goal.
"We really wanted to win. It was a quite difficult game. We went deep into ourselves to win and it shows the mentality we have," said Ayew, who has scored three goals in four games since signing from Marseille.
"We are very happy. The pass for the second goal was more important than my goal. It put us ahead."