The Gunners lost their third English Premier League game of the season on Saturday evening.
It all seem to be going to plan when Germany international defender Per Mertesacker marked his return to the starting lineup with a header in the first half.
But the home side rebounded in grand style after the restart. Troy Deeney levelled the scoreline from the spot after Richarlison was fallen in the penalty box.
Midfielder Tom Cleverley was the Watford hero of the day as he grabbed the winning goal right at he death.
In the summer, Arsenal were optimistic for the 2017-18 Premier League season after the gunners smashed their transfer record on France international forward Alexandre Lacazette.
Alex Lacazette netted 37 goals in 45 matches last season, and the Frenchman scored at least 20 goals in each of the last three League seasons.
The striker has played 90 minutes just once in the Premier league this campaign, in the opening match against Leicester. Since then he has only played: 78mins, 28, 75, 66, 83, 71, 68. That will surely affect the strikers confidence.