Petr Cech says defeat to Watford represents "a huge setback" to Arsenal's chances of winning this season's Premier League.
Chelsea's draw at Liverpool meant victory at the Emirates would have put Arsenal within six points of the leaders heading into Saturday's meeting at Stamford Bridge, live on Sky Sports.
But Watford scored twice inside 13 minutes, and an improved second-half performance brought only Alex Iwobi's reply as the Gunners ended the night nine points adrift.
Cech made no attempt to sugar-coat the result afterwards, saying: "This was a huge setback because this was the game we wanted to win.
"We wanted to put ourselves in a better position going into the weekend. I always say that at this level you cannot play 80 minutes, or 85. You have to play 90 minutes with full concentration and everybody on top of their game.
"We were second best in the first half. We made so many bad passes, we lost every challenge, we were simply second best, as I said.
"In the second half we improved, we had a lot of chances. I thought we were brilliant second half, but obviously we only scored one goal."
Cech was beaten by a deflection for Younes Kaboul's opening goal, and he said: "We had a very bad start. Obviously it gave a lot of hope and motivation to Watford's side because this was something that lifted them up.
"But then I think we didn't play well at all. It was too much for us to take in the first half and it's very hard to say why because we were prepared."