Watford Under-18s were beaten 3-1 at home to Sheffield United in their Professional Development League Two play-off semi-final.
The Hornets had the chance to take the lead before the break at Vicarage Road but Ogo Obi's penalty was saved by Aaron Ramsdale.
But the key moment in the match came just after the hour mark when substitute Brandon Mason was sent off for a challenge in the area. Jordan Hallam's penalty rebounded off the post and goalkeeper Andrew Thomas for the opener.
Watford responded immediately and Michael Folivi fired home to level the contest.
But the Blades regained the lead in the 72nd minute when Connor Hall headed past Thomas.
The visitors took advantage of the extra man and quickly added a third goal through Nathaniel Crofts to seal their victory.
Watford: Thomas; Eleftheriou, Stevens, Rowan, Rogers (Mason, 55); Ovenden, Cook (Makaka, 85), Adedeji (Ryan, 77), Pereira; Folivi, Obi
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