London - Arsenal's Petr Cech said Tuesday's shock loss to Watford was a "huge setback" to the Gunners ahead of their meeting with the Premier League leaders this weekend.
First-half goals from Younes Kaboul and Troy Deeney gave visitors Watford a quick-fire 2-0 lead in Tuesday's Premier League game and although Arsenal pulled one back through Alex Iwobi, the Hornets held on for three away points.
Defeat left Arsenal nine points behind Chelsea ahead of their trip across London to Stamford Bridge on Saturday.
"This was a huge setback," said Cech. "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."
Cech added: "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."
Tuesday's match saw Arsenal manager Arsene Wenger continue his four-game touchline ban and Cech insisted they had paid when their veteran French boss questioned if the team were "mentally ready" to face Watford.
"We had a very bad start," Cech said.
"We were unlucky with the first goal when it takes a big deflection and it goes in. Obviously it gave a lot of hope and motivation to Watford because this was something that lifted them," the Czech Republic international added.
"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.
"We were on back-to-back wins, important wins, we had the momentum after the Burnley game and obviously the Southampton game went completely our way, exactly the way we wanted."
For Watford, a first win in seven league games help them move eight points clear of the relegation zone and a delighted Kaboul said: "In the first half they couldn't live with us.
"We were on top of everything from Troy (Deeney) to Heurelho Gomes - all of us were brilliant. We did a terrific job and it will give us a lot of confidence for the rest of the season.
"Everyone on the pitch and the players who came on did a terrific job. We couldn't have started better. We could have scored more, we knew they would come back stronger and they did."