The Chelsea striker emerged from the bench after 90 goalless minutes in Salvador, and scored his side's second goal as they fought off a spirited American side, inspired by Tim Howard's superb goalkeeping display.
Lukaku's goal turned out to be the winner as Julian Green hit a brilliant volley in the second period of extra time.
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 to the tournament in Brazil.
"The coach told me I was going to come on during extra-time," he explained.
"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.
"With the potential we have, if we had been knocked out today, we would have all been very disappointed.
"The bench has made the difference in recent games, which means everyone is contributing."
