Their 4-2 victory over Portsmouth will have certainly raised Sir Alex Ferguson's eyebrows, given that it was the South Coast side that threw the destination of the title back up in the air on Saturday.
For Harry Redknapp, there must have been the fear that his side would still be dining on the glory of two days earlier when they travelled to Vicarage Road, and so it proved.
The fact Pompey took the lead only served to increase their complacency, but that shouldn't detract from the display of Aidy Boothroyd's side, who responded to Matt Taylor's 15th-minute opener in impressive fashion.
Until that point, it looked like the Watford fans might be in for a repeat of the same type of non-display that led to Saturday's 4-1 defeat at Middlesbrough that left the Championship-bound club further adrift at the basement.
There was a lack of confidence and energy but the spark Watford needed came courtesy of Pompey defender Djimi Traore, whose clumsy challenge on Tamas Priskin gave Hameur Bouazza the opportunity to level affairs. From that point on, Bouazza and his team-mates grew in stature while Pompey wilted.
It was what happened in first-half stoppage time though, that appeared to give Boothroyd's men the belief that they could go on and claim only their fourth Premiership win of the season.
David James' punch to clear a Tommy Smith cross appeared to repel the danger but the Pompey keeper didn't count on Gavin Mahon catching the rebound with a perfect left-footed half-volley that arrowed into the top corner from 20 yards.
That was the Hornets skipper's first goal of the season and the first time Watford had come from behind to lead in the top-flight.
Within five minutes of the restart, Priskin had completed a hat-trick of firsts by converting his debut Premiership goal from Steve Kabba's cross after good work by Bouazza.
From that point, the French utility forward took centre stage, netting his second after more good link play with Kabba, almost completing his hat-trick with a stunning half-volley and consistently tormenting the beleaguered Pompey defenders with his pace and trickery.
Substitute Arnold Mvuemba did bring a degree of respectability to the scoreline which, but for James, could have been even more emphatically in the home side's favour.