The West Ham side that kicked off was a mere shadow of the one that victoriously trotted off at the final whistle after goals by Nigel Reo-Coker, Darren Powell and the Ukrainian international wiped out the lightning-quick strikes of Brynjar Gunnarsson and Bruce Dyer.
And Rebrov's 58th-minute winner has certainly taken the heat off under-fire boss Alan Pardew.
After successive losses at the hands of Brighton and Millwall, the Hammers boss had been subjected to a tongue-lashing from his terrace tormentors at the New Den.
West Ham had not lost three consecutive second-flight games since December 1989, and with injuries and suspensions also limiting his selections, the pressure was quickly piling further onto Pardew who could not have seen his side get off to a worse start.
Indeed, just five minutes were on the clock when the overlapping Jermaine Darlington's left-wing cross was met by Gunnarsson whose unstoppable 15-yarder gave Stephen Bywater no chance.
And after Richard Lee brilliantly clawed out a point blank header from the on-loan Darren Powell, the Hornets hit the Hammers again on 20 minutes when the hesitant Tomas Repka failed to deal with a bouncing ball and Dyer nipped in to lob the advancing Bywater.
Predictably the exasperated East End natives had become restless, but when the consequently booked Gunnarsson scythed through Rebrov on the edge of the area as the half-hour mark approached, they found a ray of hope.
For the 30-year-old Ukrainian striker dusted himself down and unleashed a fierce, low 20-yard free-kick which Lee found too hot to handle allowing Reo-Coker the opportunity to stroke the loose ball over the line.
Then just seconds later, West Ham went a further step towards repairing that earlier damage when Luke Chadwick's corner was met by the soaring Darren Powell, who out jumped Heidar Helguson to head the home side level.
West Ham started the second period looking far more confident and cohesive than they had in that opening half.
And before the hour mark had passed, they had simply turned the match on its head.
On 57 minutes Steve Lomas' precision pass played in the overlapping Matthew Etherington whose tempting low cross was slid home at the far post by the alert Rebrov.
Both Bobby Zamora and Chadwick could have made the points safe long before Sean Dyche unluckily rapped the West Ham woodwork in the dying seconds.