Foster handed Pierre-Emerick Aubameyang his 23rd goal of the season as he dozily allowed the Gunners forward to block his kick and score.
And then Hornets skipper Deeney effectively made sure that Arsenal clinched a victory that saw them climb above Manchester United and Chelsea and back into fourth spot in the Premier League, as he was stupidly sent off a minute later, for swinging an arm at Lucas Torriera.
FA Cup semi-final hero Deeney tore his shirt off in disgust and seemed to have words with the Arsenal dug-out as he stalked off in only the 11th minute. But the damage had been done – and Gunners had their only their sixth away league win of the season, and their first clean sheet.
Foster held up hands and admitted: “I’ve said sorry to the lads. They were brilliant.
“I should have just Cruyffed him. I tried to pass it out but before I knew it he was bloody on me.
“I have to do it quicker. You have to get rid as quick as you can, even if it goes into row Z. They can’t score from row Z. He is rapid. Lesson learned.”
On Deeney’s red card he said: “Troy is not happy with it and he says it was nothing.”
Watford manager Javi Gracia said: “I am proud of my players. In my opinion it is a yellow card – but never a red. Troy is very sad. We are a team.”
Gunners rival Unai Emery admitted: “The red card helped us, but the three points is very important. A clean sheet is also very important.”
When Arsenal lost 2-1 at Vicarage Road last season, Deeney accused them of lacking ‘cojones’. He might have taken more care about his own forearm on Tuesday night rather than worrying about parts of Arsenal’s anatomy.
Watford boss Javi Gracia and Gunners chief Unai Emery were old teammates together from their playing days at Real Sociedad. When Emery was offered the Arsenal job last summer he asked Gracia his opinion, and the pair have dined together.
Even so, Emery would not have expected this kind of helping hand.
The Gunners started nervously as goalkeeper Bernd Leno sliced his clearance into touch under pressure from Andre Gray.
But then an extraordinary double disaster for the Hornets, in the space of a minute. First, Foster, recalled after Heurelho Gomes’ FA Cup heroics last week, blundered disastrously. He picked up Daryl Janmaat’s back pass, and fatefully dallied.
Aubameyang closed him down from the back unseen, and raced in to block Foster’s kick, which flew into the net.
Less than a minute later Watford were down to ten men. Deeney went into a challenge with Torriera and appeared to swing a forearm into his face. Down went the Uruguayan, linesman Richard West flagged, and referee Craig Pawson, to Deeney’s astonishment, produced the red card. Deeney, a hero at Wembley last week, had wrecked his team’s hopes.
Aubameyang then shot wide before a shell-shocked Watford fought back as Craig Cathcart saw Leno push his shot onto the post and away.
But Arsenal had the numbers now, and when Alex Iwobi shot, Foster redeemed himself partly with a fine save. Watford were still carving out chances though, and Etienne Couple forced a brilliant save from Leno with a curling free kick.
Alexi Iwobi’s cross-shot was a whisker away as Arsenal kept probing, and he crossed and Henrikh Mkhitaryan looked certain to score, only to see Foster somehow stop his point blank volley.
To their credit, Watford never gave up, and the Gunners had their own major escape as Adam Masina let fly from 30 yards – and saw his shot cannot off the angle.