The visitors gave club-record signing Isaac Success his first start and Middlesbrough's Antonio Barragan was fortunate to still be on the pitch at halftime after holding back the lively Nigerian while on a yellow card.
Success was involved in Watford's opener after going down on the edge of the area. Boro only managed to clear the ball to Holebas who, with little danger apparent, unleashed a 20-metre left-footed shot past Victor Valdes on 54 minutes.
Boro thought they should have had a penalty when Calum Chambers was challenged by Holebas in the area but could have few complaints after another toothless display in front of their home fans, who have yet to see them win in the Premier League this season.
(Reporting by Neil Robinson; Editing by Toby Davis)
