But Porter, making only his second appearance since recovering from a long-term injury, climbed off the bench to rip up the form book with a second-half header.
The Hornets, by contrast, put in a strangely subdued performance although Tom Cleverly and John Eustace came close to finding the net.
Despite their wretched away form, Derby created the early openings with Lee Hendrie seeing a shot cleared by Adrian Mariappa and Paul Connolly heading over the top.
However, the visitors were almost undone by an error from goalkeeper Stephen Bywater, who sliced a clearance straight to Danny Graham.
The striker took aim from 40 yards out as Bywater hastily back-pedalled but the ball drifted just wide.
Watford looked a shadow of the side which impressively swept past QPR 3-1 on Monday night and DJ Campbell almost made them pay for some dozy defending when he got in behind, but Scott Loach made the save.
But in first-half stoppage time the hosts almost snatched the lead, Heidar Helguson shooting through a sea of legs from a corner only to be denied by Bywater.
Just after the break a half-cleared ball fell to Campbell, who controlled it well on his knee but blazed his shot over the crossbar.
Dean Moxey then drilled a tempting cross into the area but not one of Campbell, Paul Dickov or Paul Green could apply the crucial touch.
Campbell was surprisingly taken off by Clough with half an hour to go - but it was to prove an inspired substitution.
Watford's Manchester United loanee Cleverley provided a rare moment of quality with a smart turn and shot which Bywater saved low to his left.
But when the goal finally arrived it went to the visitors, after Mariappa had fouled Porter.
Hendrie swung in the free-kick and Porter got ahead of his marker to nod the ball past Loach.
Eustace was inches away from an immediate leveller when his shot fizzed over and Don Cowie also drove too high as Derby held out.