Watford sacked Javi Gracia because he was unable to fix their leaky defence and reluctant to play several new signings, Sky Sports News understands.
Owners Gino Pozzo and his father Giampaolo lost faith in the Spaniard after just a point from their opening four games left them bottom of the Premier League - and they failed to keep a clean sheet since February.
The Pozzo family are also understood to have been worried about Gracia's aversion to playing new £40m club-record signing Ismaila Sarr and free transfer Danny Welbeck.
The pair made just one sub appearance each - both during the 3-1 home defeat to West Ham - as the Hornets suffered three defeats and a draw in their first four games.
Both players could now start the clash with Arsenal, live on Sky Sports Super Sunday, after Watford brought Quique Sanchez Flores back to Vicarage Road for a second spell as boss.
The Spaniard, who was sacked by the club in 2016 despite finishing 13th and leading them to the FA Cup semi-finals, says it was a relatively easy decision to resume his time at the helm.
Flores said: "It was really easy to say yes. The moment I left Watford the possibility to come back was open because of the circumstances before I left and the good memories I have with with this team, everyone at the club and the fans.
"Three years later it's a great possibility to come back to work at this club. My passion is the same, I know the ambition of the club and I felt able to work for them again."
Watford sacked Gracia - their ninth appointment since the Pozzo family bought the club in 2012 - last week just months after he took them to the FA Cup final and just shy of a year since he signed a new four-year contract.