A bland tie in the East Midlands was settled by the in-form forward, who made the most of a rare piece of slack play from the hosts to fire his side into the next round of the competition late on.
© Getty Images
The best opening of a quiet opening 45 minutes fell to Jamie Ward, who could only place the ball wide when capitalising on some sloppy Watford defending and bursting clean through on goal.
That proved to be the hosts' only real opening of note throughout the match, with the three shots on target each being provided by a Watford side who largely struggled to find any sort of rhythm to their play.
Adlene Guedioura, brought back into the Hornets' starting lineup as one of seven changes this afternoon, tested Dorus de Vries with the first shot on goal 30 minutes in, but the Dutchman was equal to the rather tame attempt.
Ward's wayward attempt in the dying stages of the first half certainly provided Forest with some hope for the remainder, but they struggled to really push on and found themselves playing second fiddle following the restart.
Full debutant Amrabat was a little wasteful after being picked out by a smart Anya pass down the right, flicking the ball into the welcoming hands of De Vries, but Watford were now enjoying some prolonged possession in the attacking third.
Amrabat was proving to be the game's most lively player, firing in a volley from the edge of the box which this time De Vries had to palm to safety with the first real stop of the match.
Visiting manager Quique Flores sparked into action with 20 minutes left to play by introducing leading marksman - and eventual matchwinner - Ighalo, yet the chances continued to remain at a premium at both ends.
Costel Pantilimon was another Watford player to be handed his debut in the East Midlands, and his first piece of business in a Hornets shirt finally arrived late on when keeping out Eric Lichaj's speculative shot.
Sanchez Flores looked to have gone for a defensive reshuffle in the closing minutes of the game, but it was his side who would snatch a late winner following a defensive error at the back from Forest.
A cross from the right was not cut out, allowing Ighalo to smash the ball home from close range to make it 10 goals in his last 15 outings in all competitions.
Forest pushed desperately for a leveller in the remaining few minutes of added time, yet Nelson Oliveira's saved shot proved to be the last opening of the contest to bring the Reds' 13-game unbeaten run to an end.