Watford, a goal to the good through Marlon King when the veteran defender was shown the red card, doubled their lead soon after the break. United pulled one back quickly but a second of the night for King and a fourth from the substitute Hameur Bouazza confirmed a result that provides a boost for both Watford and Leeds, although both still have 11 points to make up to deprive Neil Warnock's team of second place.
Boothroyd could not have had a better start as Watford took a sixth-minute lead. King offered a clever lay-off to Darius Henderson on the left side and the striker's cross was met by an equally deft header from Chris Eagles, beating Paddy Kenny at the near post.
There seemed ample scope for Warnock's side to fight back but the odds tipped further in Watford's favour with the dismissal of Unsworth after 25 minutes.
Eagles was involved this time as the victim of what rugby league watchers might describe as a hand-off, the defender applying a firm push to his opponent's face as they rose from a tangle of legs.
Warnock clearly felt it was harsh, but if the referee, Andy Hall, saw it as an aggressive act he probably had little option. Eagles and Ade Akinbiyi were booked after squaring up to each other afterwards.
The red card had a galvanising effect on United and when Paul Ifill stole in behind Malky Mackay to stab home Craig Short's pass in the 58th minute the roar from the crowd would have been greater had King not doubled Watford's lead only two minutes after the restart, going past a dilatory Leigh Bromby on the end of Matthew Spring's pass before beating Kenny to record his 14th goal of the season.
Even with 10 men, United put Watford under heavy pressure but the home side dealt poorly with a corner and King put away his second of the night. Substitute Bouazza capitalised on another Bromby error to stretch the lead to 4-1.
Sheffield United (4-4-2) : Kenny; Geary (Morgan, 59), Short, Bromby, Unsworth; Ifill, Jagielka, Tonge, Armstrong; Akinbiyi, Kabba (Shipperley, 83). Substitutes not used: Flitcroft, Quinn, Gillespie.
Watford (4-4-2) : Foster; Doyley, DeMerit, Mackay, Stewart; Eagles (Chambers, h-t), Spring (Bangura, 53), Mahon, Young (Bouazza, 77); King, Henderson. Substitutes not used: Chamberlain (gk), Carlisle.
Referee: A Hall (W Midlands).
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