Watford went straight down the other end and King tested De Gea after a mazy run down the left, before Ronaldo was then denied by Foster one-on-one from point-blank range.
As the clock ticked past the midway point in the second period, Nemanja Matić swiped wide with a speculative attempt for United, before skipper Maguire was shown a second yellow card for a poorly-judged tackle on Tom Cleverley.
Brazilian youngster João Pedro injected energy into the game off the Watford bench and went close to adding to his side’s lead with a glancing header in the 85th minute, while fellow sub Cucho Hernández similarly proved to be a thorn in United’s side.
And after going so close to opening his account for the season, João Pedro would indeed get his maiden Premier League goal before the death, lashing home from a tight angle past De Gea to put the result beyond doubt.
With five minutes added to the 90 there was still time for Watford to treat the Vicarage Road faithful to a fourth, with Dennis putting gloss on the scoreline with a smart strike across the face of goal, clinching three valuable points in style.