Rooney became the third-highest scorer in United's history when he bagged a brace against West Ham on Saturday.
Rooney has scored 212 goals in 436 appearances, leaving him just 25 short of Denis Law and 37 shy of the man who tops the charts, Sir Bobby Charlton.
When Rooney signed a new five-and-a-half-year contract last month, Moyes challenged him to become a United great like Charlton and Law and he believes the striker has already achieved that accolade following 10 glittering years at the club.
"If you are third top scorer of a club like Manchester United you already are a legend," the United manager said of the man who has scored 15 goals for the club this term.
"As long as he doesn't pick up any serious injuries he has a real good chance of breaking Denis Law's and Sir Bobby Charlton's (records) and that would be some feat."
Rooney's first strike at Upton Park is a strong contender for goal of the season.
The United striker brushed off James Tomkins' challenge before lobbing West Ham goalkeeper Adrian from just inside the Hammers half.
The goal evoked memories of David Beckham's goal against Wimbledon from a similar distance in 1996.
Moyes is looking for a similar performance from his star striker on Tuesday night when neighbours City head to Old Trafford for the 167th Manchester derby.
"I don't know if he'll be the key man because this game has got many top-class players in it, but he is certainly one of them," the Scot said.
"His form throughout the season has been very good and his goal against West Ham capped it all off.
"He has a real appetite back again.
"On Saturday he showed what he is about. He has a big energy back again.
"Everyone who knew Wayne Rooney from old in his early days would say one of the biggest things (about him) was that he had big energy and a big heart.
"And if you get Wayne playing like that well the rest of his football takes care of itself."
Rooney certainly has the pedigree to be a pivotal player in the match against the team Sir Alex Ferguson famously branded United's "noisy neighbours".
The free-kick the England striker scored against in last September's 4-1 defeat to City made him the top scorer in Manchester derbies.
Perhaps his most famous strike in a United shirt - before Saturday at least - came when he scored an acrobatic winner in a 2-1 victory against City.
Three years on from that bicycle kick, he has become one of the elder statesman at Old Trafford.
Rooney has been heavily tipped to succeed Nemanja Vidic as United captain when the Serb departs for Inter Milan at the end of the year and Moyes thinks the former Everton striker showed his leadership skills on the pitch at West Ham.
"Wayne led the team well," said Moyes, who handed Rooney the captain's armband in the absence of Vidic and Patrice Evra.
"He has a real appetite back again and I thought he took responsibility for the team a little bit on Saturday as well, and saw it as part of his job."