Leicester boss Craig Shakespeare said the referee "guessed" over the penalty decision that led to City's defeat at Atletico Madrid in the first leg of their Champions League last-eight tie.
Jonas Eriksson awarded a spot-kick for a foul on Antoine Griezmann, with replays showing the offence took place outside the box.
Griezmann scored the penalty to give Atletico a 1-0 victory in Spain.
"It is a really disappointing decision by the referee," said Shakespeare.
"It's a key moment in the game. He has to get that one right, he can't guess on those things.