The Chelsea forward entered the pitch at the beginning of extra time with the game still goalless, setting up Kevin de Bruyne for Belgium's first goal before adding a second soon after.
Although Julian Green pulled back a late goal for the USA, the Belgians held on to claim the win and progress to the quarter-finals.
Lukaku had been criticised for his displays at the tournament in the group stages, but Howard admits that his former Everton teammate was the deciding factor.
"They brought big Rom (Lukaku) on and he was a handful, he ran at us, scored a goal, created chances for other guys," Howard told reporters. "Big Rom changed the game."
Howard made 15 saves during the match, more than any other player in a single game in World Cup history.