Rooney scored in England's recent home wins against Montenegro and Poland, which secured the national side a place at next year's FIFA World Cup™ finals in Brazil.
The 27-year-old forward has also scored five goals in his last six games in all competitions for United to take his tally for the season to seven for club and country, having played three games at the start of the campaign without a goal.
But ahead of United's English Premier League clash with Southampton at Old Trafford on Saturday, Moyes bristled at suggestions Rooney had only lately rediscovered his best form.
"I saw it in the first day of pre-season," Moyes, who supervised the start of Rooney's professional career when Everton manager, told a news conference on Friday.
"I saw him in great form and in great fettle. You can see by his form and the goals he's scoring that hopefully what we were saying from the start is right," insisted Moyes, in his first season with the Red Devils.
Moyes said he was not surprised Rooney was able to provide such a significant contribution during the decisive phase of England's World Cup qualification campaign.
"So from that point of view I think that's what would be expected. He takes on the responsibility and scores the goals when required and I think that's what he has done. He's done that from a young age. But the age he is at now, he's got that maturity which also shows in how he plays, and the condition he is in and the way he's played for both club and country."
De Gea doing fine
At the other end of the pitch, Moyes said United were reaping the rewards of goalkeeper David de Gea's tough introduction to English football. De Gea made a number of high-profile mistakes early in his career at Old Trafford but has established himself as the club's number one.
And Moyes said the 22-year-old Spaniard, who is understood to be close to signing a two-year contract extension, has the potential to get even better.
"I think that David has had to come here and prove himself at Manchester United," Moyes explained.
"Alex Ferguson bought him young and played him at times when it was difficult. I think to play a young goalkeeper, or a young centre-back, these days is brave because they are the two hardest positions on the pitch for young players.
"But Manchester United doing that over the last couple of years has showed what you can get. He's a top goalkeeper and he will go on in time to be one of the best there is, I've no doubts about that. He is still young for a goalkeeper and most people if you look around the league tend to have goalkeepers that are a bit older in age."
United will be without midfielder Tom Cleverley for the visit of Southampton - who unusually find themselves four points in front of the reigning champions, languishing in ninth place - following a calf injury which forced him out of England duty.