Mexican Miguel Layún entered into the game on its last 18 minutes, but he was unable to save Watford from a defeat before Manchester City by a score of 2-0 on the fourth date of the Premier League.
Layún entered as a substitute for Ighalo in order to reinforce the visitors, but he was unable to lift them from the floor.
Raheem Sterling scored his first Premier League goal from six yards with a shot into the bottom right corner off a cross from Bacary Sagna in the 47th minute.
Fernandinho added a second goal less than 10 minutes later at Etihad Stadium with a firm strike after an assist from David Silva.
City won its 10th straight league match to set a club record. It leads the league with 12 points from four games this season, having scored 10 goals and conceded none.