But Huddersfield, which became only the second club after Derby in 2008 to be relegated from the Premier League in March, lost its 14th league game at home this season to match Sunderland's mark set in the 2002-03 and 2004-05 seasons.
Grant's late header at least lifted Huddersfield up to 20 league goals this season, level with Derby's record low in 2007-08. Huddersfield was a goal down in the fifth minute following an unfortunate error.
Jonathan Hogg slipped after taking goalkeeper Jonas Lössl's pass 20 yards (meters) from his penalty area. Although Terence Kongolo's last-ditch tackle denied the surging Abdoulaye Doucoure, Deulofeu struck from the rebound.
Deulofeu sealed victory for Watford in the 80th minute, sidefooting home after Lössl had saved Ken Sema's initial shot. Huddersfield gave its fans something to cheer in the third minute of stoppage time when Karlan Ahearne-Grant nodded home Chris Lowe's headed pass.
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