Hayes: Netted the winner at Vicarage Road
Adrian Boothroyd's side have won just one of their last 13 games and, after being eight points clear of the field in November, could now fail to even make the play-offs.
On Saturday, the Hornets played with the body language of a side who had given up on dreams of playing in the top flight next season and when Hayes headed home Jonathan Forte's cross with over 20 minutes remaining, there was no response.
Watford's poor run of form had certainly affected the Vicarage Road atmosphere as the game was played in a pre-season hush until the goal.
Forte nearly took advantage of a mix-up between Lloyd Doyley and goalkeeper Richard Lee to poke in but Danny Shittu recovered well in a dire first 45 minutes.
Lee was not having the best of afternoons and summed up the general lethargy on display in the first half when, after taking 20 seconds to weigh up his options from a goal-kick, he shanked it into the stands.
The hosts were in need of a drastic improvement after the break and John-Joe O'Toole burst into the area within seconds to force a corner.
But it was the visitors who went close when a corner at the other end was headed back by Izzy Iriekpen and Forte lashed a shot into the side-netting.
Substitute Matt Sparrow was played in by Hayes but delayed his shot long enough for Shittu to make a challenge.
Ben May then headed an Ian Morris corner wide as Nigel Adkins' men pushed on and Forte close to an opener in the 57th minute.
Sparrow ran down the right before cutting the ball back to Hayes, who crossed for Forte to loop a header over Lee but onto the roof of the net.
On-loan Chelsea midfielder Cork was now completely running the midfield as the nervous hosts looked clueless.
Boothroyd had to change things and decided to withdraw the anonymous Tamas Priskin and Jobi McAnuff for Lionel Ainsworth and record signing Nathan Ellington.
But again it was the side who will be playing in League One next season who looked dangerous and Morris drove in a shot which deflected off O'Toole and narrowly wide.
And Adkins' men took a deserved lead in the 69th minute through Hayes.
Forte shrugged off Lloyd Doyley with an aggressive run down the left and, from his driven cross, Hayes was on hand to head past Lee from six yards.
Boothroyd's side pushed men forward in search of a goal but could not find one and now need a win at Blackpool next weekend to avoid missing out on the play-offs altogether.