The Premier League newcomers will be taking on a Conference South site when they make the short trip to Hertfordshire on Wednesday night.
Watford starting XI: Gomes (GK); Anya, Angella, Cathcart, Pudil; Watson, Capoue; Forestieri, Abdi, Ighalo; Deeney (c)
Subs: Arlauskis (GK), Gilmartin (GK), Prodl, Hoban, Belkalem, Young, Doherty, Paredes, Martin, Byers, Murray, Smith, McGugan, Dyer, Fabbrini, Lewis, Vydra
There have been big changes for the newly promoted Premier League club since their last game in May, with Slavisa Jokanovic leaving the club after taking them back to the big time, and a number of transfers coming in over the close season.
Goalkeeper Giedrius Arlauskis, Austrain international defender Sebastian Prodl, and Jose Holebas have all arrived while Matej Vydra has signed permanently after two years on loan at Vicarage Road. Etienne Capoue signed this week from Tottenham to break the club's transfer record and makes his debut, and any number of the other players could also be set to feature in the opening match of Flores's reign.
St Albans are a side not too far away from Watford, and should be a steady introduction to pre-season matches considering they're in the Conference South.
The game should be interesting to see what formation and tactics the former Atletico Madrid manager is looking to get his new side playing ahead of their return to the top flight.