In the first top-flight encounter between the teams since February 1988, Watford prevailed through Deeney’s 30th-minute penalty and an injury-time goal by his strike partner Ighalo.
Deeney now has five goals for the campaign and Ighalo nine, and their goals have put Watford well on their way to achieving the minimum objective of avoiding relegation.
Victory took Quique Sanchez Flores’s side up to ninth place in the standings and kept Norwich, who have won only once in 10 league games, three points above the relegation zone in 16th place.
Seeking to improve on a return of one win in nine league matches, Norwich manager Alex Neil dropped goalkeeper John Ruddy for Declan Rudd and also brought Alexander Tettey and Nathan Redmond into his starting XI.
Tettey was making his return from a groin injury, but it proved an unhappy comeback for the Norway midfielder as his foul on Ighalo gave Watford an opportunity to take the lead from the spot.
Deeney made no mistake with his penalty, sending Rudd the wrong way to become the first Watford player to score in four successive Premier League games.
Ighalo thought he had doubled Watford’s lead when he neatly converted a headed pass from Allan Nyom, but he was denied by an incorrectly raised offside flag.
The Nigerian soon resumed his torment of the Norwich back four and had an effort parried by Rudd shortly before half-time after a slick nutmeg on Ryan Bennett.
Neil made a bold change at the interval, sending on striker Dieumerci Mbokani for winger Redmond, and saw his side squander a presentable chance early in the second half when Robbie Brady blazed wide.
Mbokani narrowly failed to make contact with a cross from Brady, but Ighalo continued to prove a pest at the other end, twice being thwarted by Rudd and twice shooting wide.
The Nigeria international’s perseverance eventually paid off in the first minute of stoppage time when he held off Sebastien Bassong and placed a low shot past Rudd before tearing off his shirt in celebration.
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