Watford manager Javi Gracia changed his entire starting side from the team that drew 3-3 at Bournemouth, with Venezuela forward Adalberto Penaranda making his Hornets debut in the FA cup win over non-league Woking on Sunday.
But hopes of Woking, 110 places adrift of the Hornets, enjoying a shock win suffered a setback when Will Hughes curled in a 13th-minute shot from the edge of the area.
Watford made it 2-0 16 minutes from time when Troy Deeney scored from close range.