Arsenal goalkeeper Petr Cech concedes that the 2-1 defeat to Watford on Tuesday evening is a major set-back ahead of the crucial English Premier League fixture against Chelsea.
The Gunners travel to Stamford Bridge on Saturday after a shock result against the Hornets, after failing to overturn two early goals from Younes Kaboul and Troy Deeney.
A similar result against the Blues this weekend will cut Arsene Wenger's side 12 points adrift from the title race and possibly drop out of the top four position.
"This was a huge setback because this was the game we wanted to win," he told Arsenal Player after the game.
"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."