Iheanacho opened the scoring in the first half when he pounced on a loose ball to round the goalkeeper.
He added his second of the day with a close range header to continue his rich vein of form.
The former Manchester City Striker has now scored 20 goals in English Cup competitions.
Iheanacho has scored 7 goals in his last four matches for Leicester City.
He was also voted the man of the match following his impressive display, he had a hand in all the goals against Manchester United.
The 24-year-old has scored 12 goals and provided five assists in 27 matches in all competitions for Leicester City this season.