Younes Kaboul and Troy Deeney put the Hornets two up inside 15 minutes before Alex Iwobi pulled a goal back for the Gunners, but Arsene Wenger’s side could not find the equaliser.
And Cech believes the defeat was down to the sluggish opening 45 minutes.
Arsenal star Petr Cech blamed the defeat to Watford on a poor first half
This was a huge setback because this was the game we wanted to win
Arsenal star Petr Cech
“We had a very bad start,” he told Arsenal Player.
"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’s side because this was something that lifted them up. But then I think we didn’t play well at all.
"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.
Alexis Sanchez looks absolutely distraught after Arsenal's defeat to Watford
Tue, January 31, 2017
Alexis Sanchez looked absolutely distraught after Arsenal's defeat to Watford
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Arsenal lost 2-1 to Watford on Tuesday
Arsenal's Petr Cech and Alex Oxlade-Chamberlain look dejected following the defeat to Watford
"We had chances but the keeper made some very good saves, we hit the bar and we could say we were unlucky in that way. But for the first-half performance, we actually didn’t deserve a little bit of luck.
"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.
"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.
"This was a huge setback because this was the game we wanted to win and we wanted to put ourselves in a better position going into the weekend."