The passion and excitement that has carried his team so far in this competition - and could prove to be his only passport into Europe next year - vanished without a trace.
The Spaniard will have just cause to be worried about the second leg at Vicarage Road and will no doubt be tentatively scouring the video tape for evidence of his team's complacency.
Both sides created clear chances within the first few minutes of the game but Watford quickly emerged as the most hungry for the win.
Watford were cheered on by a travelling sea of yellow shirts, which were occupying twice the usual allowance given to away fans.
Brynjar Gunnarsson rewarded their enthusiasm early on when he turned Steven Gerrard inside out with a clever back-heel before side-stepping Steve Finnan and laying the ball to Paul Devlin. But his run was thwarted by Jamie Carragher.
The Hornets definitely played the more attractive football during the opening stages of the match and at times Liverpool looked as if they were two leagues below them.
Watford's fans' infectious backing was almost rewarded with a goal in the 18th minute when Devlin delivered a pin-point pass towards an unmarked Neal Ardley but his powerful header went straight into the waiting hands of Jerzy Dudek.
Liverpool looked sluggish and couldn't build up any kind of fluid passing moves, with most balls falling at least two feet behind the intended player.
And it was the Reds who found themselves fiercely defending their goal again when Jermaine Darlington won the ball just inside the area and after a few fine twists was unlucky to see his shot blocked by Finnan.
The only ray of hope for the home fans came on the stroke of half-time when Captain Fantastic Gerrard picked up the ball on the halfway line.
He delivered an unbelievable 40-yard pass between two yellow shirts straight to the feet of Neil Mellor, but he couldn't convert the chance.
Benitez looked furious as he left the pitch at half-time and his players must have been dreading nothing less than a Spanish Inquisition. Whatever he said seemed to have the desired effect as Liverpool looked slightly more awake for the second half with Gerrard leading the charge.
He dazzled the defence before unleashing a clever chip which beat Paul Jones' outstretched glove but drifted over the bar.
In Liverpool's next attack he drove hard towards goal but this time he played the ball off to Mellor, only for him to stumble and get dispossessed.
Mellor, for all his goal heroics in the previous rounds, was simply not good enough on the night and was substituted by Milan Baros in the 52nd minute.
The change immediately played dividends as the Czech hitman's quick pace tested Watford's defence.
Liverpool's resurgence had Watford back on their heels and Gerrard knew just how to take advantage when he broke the deadlock in the 55th minute.
Baros created the goal by steering a low ball across the goalmouth which Jones did well to parry.
But it was only cleared as far as Gerrard and he unleashed a thunderous effort which Jones had no chance of stopping.
Watford's Neil Cox bravely tried to head the ball off the line but he merely served to deflect the ball into the middle of the net in a more dramatic fashion.
Jones was unlucky to be caught out by a gaping hole in his defence and for the majority of the match he provided a solid wall against his old Anfield team-mates.
Liverpool were then content to sit on their one-goal buffer and hold the ball up in midfield.
The Hornets had one last-gasp chance when Premiership prospect Heidar Helguson's shot was blocked by the long legs of Djimi Traore.