On a rare blank day for the two strikers, Leeds United’s unfortunate defender Scott Wootton did the job for them, carelessly turning a cross into his own net.
That was sufficient for a third successive laboured FA Cup win; Newcastle and Nottingham Forest had been beaten 1-0 in previous rounds.
It seems that Watford, in cricket parlance, are getting them in singles – all the way to Wembley, perhaps, in a season when they have confounded expectations.
Their impressive manager, Quique Sanchez Flores, made 6 changes to the side who have performed so well in the Premier League – leaving Ighalo on the bench until midway through the second half.
He was missed, not least because, although January signing Nordin Amrabat is a clever little player with good movement, he is not a natural centre-forward.
He would regularly drift out wide, only to find no one in the penalty area.
The surprise was that Leeds, defending solidly in the first half without producing anything themselves, conceded a goal before Ighalo arrived.
Eight minutes into the second half, Ben Watson, Wigan’s 2013 FA Cup Final hero, put over an outswinging cross and, with goalkeeper Marco Silvestri apparently unsure whether to come for it or not, a confused Wootton side-footed the ball into his own net.
Had Etienne Capoue not had an effort disallowed for a foul by Deeney, Watford could have relaxed a little but in the event Leeds, 16th in the Championship, were almost as toothless in a wind-assisted second half as in the first.
With striker Chris Wood still unfit, they failed to score for a third successive game.
Steve Evans, nearly as vocal on the touchline as the 4,500 visiting supporters, admitted: “We know where we need to strengthen. Defensively we kept a good shape today but offensively we have [given] the ball away in sloppy areas and when you see Watford attack, their one-and two-touch was what you expect from the Premier League.
“I think the performance was good. It was the cruellest way ever to go out of the FA Cup – it was a terrible goal to concede.”
The fans’ expectations wear most Leeds managers down in the end but Evans insists: “If we get the support to bring in the players we want, we’ll be in the promotion mix next season.”
He was also generous in praise of Flores – “a great addition to English football” – who said: “I’m really pleased with the performance of the team.”
"Leeds were very strong in defence and organised. We needed more players to go [into] the box and second half we corrected that and controlled the midfield.”
Overall, however, it was a poor game that ended with a paltry total of one shot on target by each team – own goal not included – and Leeds supporters chanting a familiar, if impotent refrain at their owner: “Massimo, time to go.”
Watford: (4-3-2-1) Pantilimon; Paredes, Britos, Cathcart, Holebas; Capoue (Anya, 88), Watson, Suarez; Abdi (Guedioura, 77), Deeney; Amrabat (Ighalo, 67).
Leeds United: (4-1-3-2) Silvestri; Wootton, Bellusci, Bamba, Taylor; Diagouraga (Botaka, 86); Dallas (Mowatt, 60), Cook, Bridcutt; Doukara, Antenucci (Erwin, 83).
Referee: Michael Oliver
Man of the match: Watson (Watford)
Match rating: 5/10