Watford struggled through to the second round of the Carling Cup thanks to a lone strike by Andrew Ferrell at Vicarage Road.
Signed from Newcastle United during the summer, Ferrell celebrated his debut with a goal scored four minutes after coming on as substitute.
The ball was played into Hameur Bouazza by Neal Ardley. The striker slipped the ball into the path of Ferrell who beat the keeper with a rising drive from 20 yards out.
The goal spared Watford's blushes as until the breakthrough they had struggled to assert their authority over a side ranked two divisions lower.
Watford had a couple of half chances in the first period. Paul Mayo saw a fierce shot deflected on to the post and out with nobody following up.
Then Ashley Young tested John Ruddy with a low shot after working himself a shooting opportunity.
United, who been forced to make two first-half substitutions, played the better football but were unable to find a way through.
Watford stepped up the pressure with extra time looming. Bruce Dyer headed just wide following a cross from Mayo then Neil Cox failed to get hold of his shot when meeting a cross from Paul Devlin.
At the other end Jermaine Easter came close to giving United the lead but his powerful drive just missed the target.