Arsenal manager Arsene Wenger brushed off his side's 1-0 loss to Swansea City as "an accident" and challenged his players to atone when they visit Manchester United on Sunday.
Arsenal dominated Monday's game at the Emirates Stadium, but having been kept at bay by their former goalkeeper Lukasz Fabianski, they capitulated in the 85th minute when David Ospina was unable to keep out a header from Swansea substitute Bafetimbi Gomis.
United can dislodge third-place Arsenal from the automatic Champions League qualifying positions if they beat them at the weekend and Wenger's annoyance was plain during a post-match press conference in which he said Swansea had "refused to play completely and just defended".
"We have still a game in hand," said the Frenchman, whose team play host to Sunderland on May 20 before tackling West Bromwich Albion on the season's final day.
"What is crucial is to respond on Sunday and show it is an accident today (Monday). That is the most important (thing)." Wenger said.
Arsenal had won nine of their previous 10 games, only dropping points in a goalless draw with Chelsea, and fielded an unchanged team for the fifth game in succession for the first time since January 1994.
It took until the 59th minute for them to register a shot on target and although they mustered eight more, there was no way past Fabianski, who saved from Olivier Giroud, Santi Cazorla, Theo Walcott, twice, Nacho Monreal, Aaron Ramsey and Alexis Sanchez.
"We scored plenty of goals until recently," added Wenger, who felt that his forwards had been guilty of "rushing" their finishing.
"In the last two home games we didn't score and that's of course frustrating when you have so much of the ball, as we did tonight, and you play with such accuracy around the box."
"But they defended well. They came just to defend, with four defenders and six midfielders. They defended, that's it. If you win, you're fantastic. If you lose people say to you, 'Why did you not play?'" Wenger said.
Having beaten Arsenal 2-1 at the Liberty Stadium in November, Swansea became only the second team to do the double over the London club and Manchester United in the Premier League era after West Ham United (2006-07) and Chelsea (2009-10).
The surprise loss puts Manchester United back in contention for a third place finish, provided they can beat Arsenal in the next game. A third place finish would mean a confirmed entry into next season's Champions League.
Europa League chance
Swansea manager Garry Monk took exception to Wenger's criticism of his side's approach and said their victory proved that they have become a more multi-faceted team.
"If we didn't play, I don't know how we got three points," he said.
"If you watched all our games, we've won games totally dominating possession. The number one thing for me is that we try to do that in as many games as possible. The players know that." Monk said.
"But there are times where you have to be adaptable and we've shown that time and time again this season,"
"That's the double over Arsenal. We've done the double over Manchester United. So if people don't like it, it's not my problem." Monk said.
Swansea are now just one point off seventh place, which will yield a Europa League place if Arsenal beat Aston Villa in the FA Cup final, and two points off six place, which is a guaranteed Europa League berth.