Rooney found the net in World Cup wins over Montenegro and Poland to help guide England to the World Cup finals this week.
The striker has also scored five times in his last six matches for United but Moyes, who had to deal with continued questions over the player's supposed discontent at Old Trafford, knew from pre-season to expect a player at the top of his game.
"I saw him like this on the first day of pre-season yet I had to keep telling everyone that was the case because the media thought it was different," said Moyes.
"I saw him in great form. He was in great fettle. His training was fantastic from day one.
"But people kept saying different things and I found it difficult to prove it different.
"You can see by his form and the goals he is scoring that what I was saying was right.
"He is certainly a main player for Manchester United and he is one of the main players for England; that is what we expected.
"He takes the responsibility, scores the goals when it is required. He has done that from a young age.
"But the age he is at now he has that maturity which shows in how he plays, the condition he is in and the way he is playing for both club and country."