Substitute Goetze, who until the final had had a disappointing tournament, struck the only goal seven minutes before the end of extra time to clinch Germany's fourth World Cup.
"I told him 'you go out there and show the world that you are better than (Argentina captain) Lionel Messi and that you can decide the game tonight'," Loew told reporters.
Four-times world player of the year Messi was looking for a first World Cup victory that would place him among the world's greatest players.
But Goetze stole the show.
"That's what I told him and I had a good feeling," said Loew, who brought on the attacking midfielder two minutes before the end of normal time for striker Miroslav Klose.
The 22-year-old paid back his coach's trust, volleying in his second goal of the tournament after spending the past few games on the bench.
"Goetze is a wonder boy who has these immense abilities, this outstanding skill," Loew said. "He can always decide a game and he scored a great goal today."
For man-of-the-match Goetze, it had not been an easy tournament after initially failing to live up to expectations.
One of Germany's most talented and promising players, he scored in the group game against Ghana but then was not used much.
"It was not an easy year, not an easy tournament for me," said the softly-spoken Goetze.
"But I am just happy that the team won this title with me. I kept training with the team and every player here deserves it," said Goetze.