Arsenal returned to the top four in the Premier League when the 10-man Watford was the tonic for his absence illnesses.
With questions beyond their bad shape on the road, the Gunners profited from a rare Ben-Foster mistake and a careless moment from Troy Deeney, who deposed the Watford skipper, as Arsenal still after a 1-0 win worked hard.
The hosts began the game with the front foot, as they used the hopeless balance of Arsenal off the Emirates Stadium.
But two of their outstanding players recently changed the game in favor of Unai Emery's team in less than two minutes.
Foster – so faithful in Watford's goal of the season – inexplicably distorted himself when clearing his lines and could hit the ball only in the foot of Pierre-Emerick Aubameyang when he tore it down and the ball flashed into the empty net.
If that was not enough, Captain Deeney was released a moment later when he caught Lucas Torreira with a stray forearm.
Referee Craig Pawson worked with his assistant before presenting the red card. The angry Deeney took off his shirt and exchanged with the Arsenal Bank when he finally left.
Arsenal had won only five away league games throughout the season before moving to the finalist of the FA Cup. Nevertheless, he was able to secure the three points that were needed to bring both Chelsea and Manchester United to fourth place.
The Hornets had almost knocked back when Craig Cathcart hit a free-kick long ago, whose low blow was pushed onto the post by Bernd Leno.
Despite their numerical advantage, Arsenal had no idea after Deeney's departure that Konstantinos Mavropanos, who had made his first start since May, barely fought a set of set pieces and crosses into the pits.
However, the guests remained a threat during the break and Foster had to be alive to complete an Alex Iwobi ride, while Leno was there to steer Etienne Capoue's free-kick around the post.
Emery responded with the introduction of Mesut Ozil instead of Torreira at half-time, but Henrikh Mkhitaryan missed a good chance to double Arsenal's lead as Foster made a nice stop in front of the Armenians' backer.
Watford, who had just finished in the second final of the FA Cup, was still in the game and Adam Masina was unhappy when he saw a long strike against Leno's crossbar shortly after one hour.
Emery switched the formation to a back-three before returning to the system that had started the game. Matteo Guendouzi and Ainsley Maitland-Niles tried to take control of the competition.
Maitland-Niles was soon on the spot to keep Arsenal at the top, and broke in bravely to block Gray when Watford went in the last 10 minutes and his guests were back on the ropes.
Will Hughes walked over as the noise rose in Vicarage Road. Aubameyang scored a shot from a rare opening for Arsenal.
The team of Emery was exhausted despite battling a team that played 79 minutes with 10 men, as the players hugged each other on the court and had passed a rigorous test of their mentality.