Thomas Tuchel admitted that Chelsea "were not ready" for their game with Watford and fortunate to escape with three points.
Hakim Ziyech's second-half winner ensured that Chelsea remained top of the Premier League in an otherwise sloppy 2-1 victory.
With Manchester City and Liverpool also winners on a bumper night of midweek action, a win was vital for Tuchel's title-chasers, but they largely struggled to control an encounter with Claudio Ranieri's relegation-battling Watford.
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The game was halted for more than half an hour with the score at 0-0 after a medical emergency in the stands - a delay that seemed to affect Tuchel's side.
The German conceded that his side were "very lucky" to avoid dropping points.
“[It was] very sloppy and it was a very unusual performance for us,” Tuchel said after the game. “I had the feeling we were not ready.
“I have to get the team ready and obviously I didn’t prepare them for what was coming because for the first time we were completely underperforming as a team.
“We did not cope well with the first balls, we did not cope well with the pressure and we did not cope well with the second balls.
“Watford played with a lot of enthusiasm and we struggled in the basics. I don’t know why but it was like this.
We are in this together and we got a lucky win today.
The loss leaves Watford just three points clear of the relegation zone, but there was plenty to be optimistic about as Ranieri nearly produced a signature early result against his former club.
Emmanuel Dennis continued his breakthrough campaign with an equaliser on the stroke of half-time after Mason Mount had put Chelsea ahead.
It is Watford's sixth defeat in eight games and with Manchester City their next opponents at Vicarage Road on Saturday 4 December.
Ranieri felt afterwards that his side were desperately unfortunate to not earn at least a point from a game in which they registered more shots and had an even share of the ball.
“What can I say? It’s unbelievable what happened, but that’s football,” said the Watford boss.
“I’m very satisfied and very proud of my players and we must continue this way. I think we deserved at least a point, we played very, very well. I’m very proud of my players, that’s it.”
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