The Millers’ huge clash with Millwall on Saturday looks even more vital after another bad day at the office at Watford.
A Odion Ighalo brace and a Troy Deeney strike condemned Rotherham to a heavy defeat at Vicarage Road as their relegation worries deepened.
The Millers remain two points clear of the bottom three but will be dragged in should the lose to the Lions on Saturday.
They had made a lively start to their trip to Vicarage Road but the final ball let them down against promotion chasing Watford.
Matt Derbyshire, Ben Pringle, Jack Hunt and Ritchie Smallwood all fired in dangerous crosses but the Millers failed to take advantage.
Then a Kari Arnason howler let in Ighalo to give Watford the lead.
Arnason stumbled in claiming a back pass, allowing Ighalo in to steal the ball and round Adam Collin to slot into an empty net with 18 minutes gone.
On form Matt Derbyshire had a chance to draw Rotherham level almost immediately. The forward tried to lift the ball over Huerelmo Gomez but could only flash the ball across the face of goal.
Arnason atoned for his error after Watford took the lead, beating the advancing Miguel Layun to the ball and clearing.
A similar slip to that of the Icelandic defender from Joel Ekstrand almost let in Derbyshire but he was quickly closed down.
Almen Abdi tested Collin with a shot from distance while the Millers keeper also had to be alert to deny Troy Deeney seconds later.
From the subsequent corner, Collin again frustrated the hosts as he beat away a header from Ekstrand.
The Millers’s chances of taking anything from the game were dead and buried after Watford grabbed two goals in quick succession early in the second half.
First Daniel Tozser’s free kick proved too hot to handle for Collin and Deeney pounced to turn the ball home.
Then two minutes later, Collin again failed to hold a shot from Miguel Layun and Ighalo charged to slot in.
Tommy Hoban was so close to making it three goals in quick succession as he drilled narrowly over the bar from the corner of the box.
Abdi tested Collin’s alertness with a shot from range but the Millers man was equal to it.
Tozser curled a free kick just over the bar as Watford threatened to widen the margin.
And Abdi struck the post after a fine through ball from Ben Watson.
But there was no further damage to be done for the Millers.
Watford: Gomes, Cathcart, Ekstrand, Angella, Hoban; Layun (Munari 68), Watson, Tozser; Abdi; Deeney (Forestieri 84), Ighalo (Vydra 75).
Subs not used: Bond, Paredes, Pudil, Anya.
Millers: Collin; Hunt, Morgan, Arnason, Fryers, Green, Smallwood, Milsom (Ward 65), Pringle, Sammon (Hammill 75), Derbyshire (Clarke-Harris 88)
Subs: Loach, Broadfoot, Bowery, Richardson,
Referee: Andy Davies