In the opposite dugout will be Javi Gracia, alongside whom Emery played for one season in 1995-96 at Real Sociedad.
Watford started the better of the two teams, pressing the Gunners high up the pitch and making it uncomfortable for them to settle in the match. We competed well, we had many chances and their goalkeeper made some incredible saves.
Deeney nodded down a cross from Jose Holebas to Will Hughes who missed the target with a half volley from the edge of the penalty area.
Two minutes later, the Frenchman sent a low pass across the face of goal to the far post, where German midfielder Ozil sidefooted in an easy goal to double the lead to 2-0, a score that flattered the home side after an even, open and attacking match.
Granit Xhaka tested Watford's Foster with a stinging long-range drive that the keeper parried to safety.
Lucas Torreira, booked in the first half for a late challenge on Deeney, played in Pierre-Emerick Aubameyang whose cross was headed wide by Lacazette.
However, the 36-year-old failed to complete the ninety minutes as Cech was forced off just before half time after pulling up with a hamstring injury while taking a routine goal kick in first half stoppage time.
Emery's side travel to Qarabag in the Europa League on Thursday before facing Fulham at Craven Cottage next weekend prior to the global break.
With Arsenal struggling to build a head of steam, Watford began to take control.
Alex Iwobi also had a decisive impact off the bench as his cross was steered into the net by Craig Cathcart for an own goal in the 81st minute.
Watford had to throw men forward and Arsenal picked them off again in the 83rd minute, with Ozil applying the finishing touch to Lacazette's cross.