Arsenal boss Arsene Wenger says nerves contributed to the league leaders' 1-0 loss at arch-rivals Manchester United.
A first-half header from former Gunners striker Robin van Persie gave David Moyes's side victory, and left the visitors still looking for their first league win at Old Trafford since 2006.
"We were just too nervous in the first half," said Wenger.
"We haven't won for a long time here now. Once again it is a tight game that has been decided on a set-piece."
The match contained few clear-cut chances and Arsenal only managed two shots on target, although they did go close on a couple of occasions when Bacary Sagna's pacy deliveries flashed by his team-mates.
"We rushed our game too much and lost too many balls. In the second half we were composed, had a real go and played our usual game," said the 64-year-old Frenchman.
"We were very, very, very close a few times. The heart was in the game, the desire was in there, but maybe a bit too much in the first half. We are quite strong on defending set-pieces and I was surprised we were caught on the near post.
"We had to give a lot this week - Liverpool, Dortmund and here today. I think a draw would have been a fair result."
When asked whether he considered replacing goalkeeper Wojciech Szczesny after he suffered a head injury in the first half, Wenger added: "I listen to the doctor and if he tells me a player has to come off, he comes off.
"No matter if we have seven men on the field, I do it. You only have one life; you play 60 games a year."
Arsenal's first league defeat in six matches means they remain on 25 points at the top of the Premier League, with Liverpool in second - two points behind - and United in fifth on 20 points.