Gunners climb top four thanks to Pierre-Emerick Aubameyang match winner as Troy Deeney sees Red for an elbow on Lucas Torriera.
Unai Emery’s side have struggled for form on their travels in all competitions but were gifted three points at Vicarage Road on Monday, moving them up to fourth in the table.
Foster’s lapse in concentration saw Pierre-Emerick Aubameyang score his 18th league goal of the campaign in farcical fashion in the 11th minute.
The Hornets continued to test Leno despite Arsenal’s man advantage, with Etienne Capoue’s floated free-kick turned away by the keeper.
MATCH FACTS AND STATS
Watford (5-3-2): Foster 4; Femenia 6 (Success 59, 5), Janmaat 5.5, Kabasele 6, Cathcart 6, Masina 5.5 (Sema 86); Hughes 5.5, Doucoure 6, Capoue 6; Gray 6, Deeney 2.
Subs not used: Gomes, Britos, Mariappa, Chalobah, Navarro.
Booked: Capoue, Hughes
Red card: Deeney 11
Manager: Javi Gracia 6
Arsenal (4-2-3-1): Leno 6.5; Mustafi 6, Mavropanos (Guendouzi 59, 6) 5, Koscielny 6, Monreal 6; Torreira 6.5 (Ozil 45, 6), Xhaka 6, Mkhitaryan 5.5, Ramsey 6.5 (Maitland-Niles 68, 6), Iwobi 7; Aubameyang 7.
Subs not used: Cech, Elneny, Lacazette, Kolasinac
Goal: Aubameyang 10
Manager: Unai Emery 7
Referee: Craig Pawson 6
MOTM: Iwobi 7
