United have rejected two bids from Chelsea for Rooney this summer after former manager Sir Alex Ferguson last season claimed the striker wanted to leave.
Former United defender Neville feels Rooney would be a huge loss if he left Old Trafford and is adamant the 27-year-old cannot be sold to Jose Mourinho's side - unless David Moyes has a superstar signing lined up to bring in.
He told Sky Sports News: "Manchester United can't sell Wayne Rooney to Chelsea football club, unless they bring in a major major signing.
"You've got here absolute gold in Wayne Rooney. You're talking about someone who is proven over six, seven, eight years, to deliver season in, season out.
"Wayne Rooney in this moment in time, from what I know, he'll never drop his standards in training, he'll work his absolute backside off and that's important to know. However, there is something brewing behind the scenes, you can see that.
"Chelsea have made bids for him but Manchester United can't sell him to Chelsea, unless they go and sign (the likes of) Bale and Ronaldo and you think 'okay we can potentially sacrifice someboby', but the importance of Wayne Rooney at Manchester United is absolutely vital."
He added of the 83-times capped England forward: "He's a champion player, somebody who in the middle of October when you're four points behind in the league and going to Chelsea, and you've got Arsenal the Saturday after, he's the guy who wins you the league.
"He gets up the pitch and back, sets up goals, the one who literally will be the one who will take you towards that championship, the one who will dig in.
"Don't underestimate the value and importance of Wayne Rooney to Manchester United's success in the last seven or eight years.