Pierre-Emerick Aubameyang blocked Ben Foster’s clearance to score the only goal of the game
Arsenal did just enough to beat a 10 men Watford side to climb back into the top four of the Premier League.
A disastrous two minutes at the start of the first half saw the Hornets go down 1-0 and their captain Troy Deeney sent off.
The home side battled against the Gunners for the rest of the match and may have felt they could have got something out of the game.
Arsenal made six changes for their Premier League clash at Watford as Konstantinos Mavropanos made his first start since May.
The Gunners face Napoli in the second leg of their Europa League quarter-final and travel to Italy defending a 2-0 lead.
Unai Emery is likely to have one eye on that fixture with Mavropanos starting as well as fellow recalled players Shkodran Mustafi, Henrikh Mkhitaryan, Alex Iwobi and Bernd Leno, while Granit Xhaka recovered from a groin problem to be included.
Watford meanwhile made four alterations from their victorious FA Cup semi-final win over Wolves as Ben Foster, Christian Kabasele, Daryl Janmaat and Adam Masina came in.
Arsenal took the lead in the 10th minute through Pierre-Emerick Aubameyang.
The striker capitalised on hesitation from Ben Foster to close down the goalkeeper and block his clearance, with the ball rebounding into the hosts’ net.
Things got even worse for the Hornets just a minute later as captain Troy Deeney was shown a straight red card for throwing an arm towards Arsenal midfielder Lucas Torreira. Despite playing with 10 men, Watford troubled Arsenal throughout the remainder of the first half.
Bernd Leno was forced into saves from Craig Cathcart and an Etienne Capoue free-kick, but the Gunners held their 1-0 lead at the break.
Arsenal made a confident start to the second half after the introduction of Mesut Ozil and created a number of chances.
Henrikh Mkhitaryan had several opportunities to double their lead but failed to trouble Ben Foster.
Watford, fresh from securing a place in just their second FA Cup final, 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 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.