The Nigerian forward struck twice but it wasn’t enough for the Blues, as Sadio Mane scored a hat-trick to lead the Saints to a brilliant home win.
Iheanacho, 19, hopes his performance will have earned him a start for City’s semi-final second leg trip to the Bernabeu on Wednesday night.
But he admitted Manuel Pellegrini’s side saw their preparations dented ahead of the biggest game of their season.
“This was not good preparation, this game didn’t give us enough confidence but we’ll put it behind us as soon as we leave here and focus on the game on Wednesday,” said Iheanacho.
“We need to put it behind us, it’s gone, it’s not going to come back.
“So many things went wrong in the game, we lost so many balls and we didn’t defend well.
“We lost so we’re not happy, but we’ll put it behind us and come back stronger.”