Aubameyang pounces on Foster's error but Gunners make hard work of it
Watford 0 Arsenal 1
Arsenal moved back into the Premier League top four as 10-man Watford provided the tonic for their away-day ills.
With questions hanging over their poor form on the road, the Gunners benefited from a rare Ben Foster mistake and a rash moment from Troy Deeney, which saw the Watford skipper sent off as Arsenal still laboured heavily to a 1-0 win.
The hosts started the game on the front foot as they looked to take advantage of Arsenal's frail record away from the Emirates Stadium.
But then two of their stand-out performers of recent times turned the game in favour of Unai Emery's side in under two minutes.
Foster - so reliable in the Watford goal this season - inexplicably dallied when clearing his lines and could only smash the ball into the foot of Pierre-Emerick Aubameyang as he tore down on him, the ball flashing into the empty net.
If that was not enough, captain Deeney was dismissed moments later as he appeared to catch Lucas Torreira with a stray forearm.
Referee Craig Pawson liaised with his assistant before producing the red card, an angry Deeney removing his shirt and then exchanging views with the Arsenal bench as he finally departed.
Arsenal had won just five away league games all season before travelling to face the FA Cup finalists and still made a meal out of securing the three points required to leapfrog both Chelsea and Manchester United into fourth.
The Hornets almost hit back when a long free-kick dropped to Northern Ireland international Craig Cathcart, whose low strike was pushed onto the post by Bernd Leno.
Despite their numerical advantage, Arsenal laboured following Deeney's departure, barely dealing with a barrage of set-pieces and crosses into their box as Konstantinos Mavropanos - making his first start since May - particularly struggled.
The visitors remained a threat on the break, however, and Foster had to be alive to push an Alex Iwobi drive behind, while Leno was then on hand to brilliantly turn an Etienne Capoue free-kick around the post.
Emery reacted by introducing Mesut Ozil in place of Torreira at half-time but it was Henrikh Mkhitaryan who missed a fine chance to double Arsenal's lead as Foster made a fine stop from the Armenian's back-post effort.
Watford, fresh from securing a place in just their second FA Cup final - they'll face Manchester City in the Wembley showpiece - were still in the game and Adam Masina was unfortunate to see a long-range strike crack against Leno's crossbar just after the hour.
Emery switched formation to a back-three before soon reverting back to the system which had started the game, bringing on Matteo Guendouzi and Ainsley Maitland-Niles in an attempt to wrestle control of the contest.
Maitland-Niles was soon on hand to keep Arsenal in front, bravely sliding in to block Gray as Watford headed into the final 10 minutes with their visitors again on the ropes.
Will Hughes headed over as the volume inside Vicarage Road rose, Aubameyang then flashing a shot wide from what proved to be a rare opening for Arsenal.
Emery's side, despite struggling against a team which played for 79 minutes with 10 men, were left exhausted at the full-time whistle as the players embraced on the pitch having survived a stern test of their mentality.
WATFORD: Foster, Janmaat, Kabasele, Cathcart, Masina (Sema 86) Femenia (Success 59), Capoue, Hughes, Doucoure, Deeney, Gray.
Unused subs: Gomes, Britos, Mariappa, Chalobah, Navarro.
ARSENAL: Leno, Mustafi, Mavropanos (Guendouzi 59), Koscielny, Monreal, Torreira (Ozil 45), Xhaka, Mkhitaryan, Ramsey (Maitland-Niles 68), Iwobi, Aubameyang.
Unused subs: Cech, Elneny, Kolasinac, Lacazette.
Referee: Craig Pawson
Man of the match: Aubameyang
Match rating: 7/10