The 20-year-old who has struggled to command a regular appearance in Pep Guardiola’s squad this season, replaced two-goal hero Sergio Aguero in the 79th minute and powered in the fifth goal before the final whistle.
With this victory, the Etihad Stadium giants face Middlesbrough in the quarter final.
Iheanacho, who has been invited for Nigeria’s friendlies against Burkina Faso and Senegal expressed his delight having a chance to play plus finding the back of the net.
''Great result for the team tonight. So blessed to get on the pitch, and the goal a bonus!! On to the next round,'' he wrote on Instagram.
Manchester City face relegation-threatened Sunderland in the English Premier League on Sunday and the Nigerian would be hoping to get a look in by Guardiola.
City are third on the English topflight log with 52 points from 25 games – 11 points off leaders Chelsea.