Rooney put pen to paper on a new five-and-a-half-year deal on Friday before wrapping up United's 2-0 victory at Selhurst Park 24 hours later.
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And Moyes insisted that tying down Rooney - who was linked with a move away from Old Trafford prior to the start of the season, with Chelsea openly expressing their interest - was a vital step forward for the club, as the Scot seeks to build on what has been a disappointing campaign so far.
"It is great news. It is a great signing for us. To try and buy a player like Rooney is going to cost you a lot," Moyes told Sky Sports.
"He's a really important player, you think about the amount of clubs who wanted to sign him. Wayne is one of the top players, he shows it week in, week out.
"We never had any intention of letting him go. Wayne has been great since day one."
Moyes also urged Rooney to keep on improving and become a key figure in the Scot's Old Trafford tenure.
"The finish (for his goal) was exquisite," he added. "It's a bit like Wayne, (having) not much time to think about it and bang, he pulls the trigger.
"We're on his case and we want him to keep at it."