PHOTOS: U.S. fan reactions | Beautiful people in Brazil
The Chelsea striker came off the bench after 90 goalless minutes in El Salvador to set up the first goal and score the second, as it fought off a spirited American side inspired by Tim Howard's superb goalkeeping display.
Having seen his place in Marc Wilmots' XI taken by Lille youngster Divock Origi, Lukaku revealed assurances from his coach allowed him to remain calm and finally make a telling contribution Tuesday night.
"The coach told me I was going to come on during extra time," he said. "As soon as I got onto the pitch, I knew that I would find the net. From my very first sprint, there was a big chance. At that moment I was very happy, because for once I did something good on the pitch.
"I perhaps put myself under too much pressure. I just wanted the team to win and for us to continue."
After dispatching the Americans, Lukaku says his side can continue on in the tournament.
"With the potential we have, if we had been knocked out today, we would have all been very disappointed," he said. "The bench has made the difference in recent games, which means everyone is contributing."
Belgium will face Argentina in the quarterfinals on Saturday.