Chelsea were a runaway success last term, lifting the title in Antonio Conte’s first season in charge of the club.
Conte has made four signings so far this summer - Willy Caballero, Antonio Rudiger, Tiemoue Bakayoko and Alvaro Morata.
The Italian tactician has been frustrated at missing out on some of his key targets to his rivals.
And Neville is convinced Matic’s exit will be felt at Stamford Bridge.
“I can't think of a reason why Chelsea would sell him [Matic],” Neville said on Sky Sports.
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Nemanja Matic joined Manchester United in a £40million move
I think the [Matic] sale at this moment in time, with that Chelsea squad, could cost them the league
Phil Neville
"If Nathaniel Chalobah or Ruben Loftus-Cheek were still there they could get rid of Matic and play a younger kid.
“You would applaud that but those players aren't even at the club now. They have weakened their own team and strengthened Manchester United, who now have a better chance of winning the Premier League than Chelsea.
"United were crying out for a central midfielder with the qualities of Nemanja Matic. He is probably the best in England alongside N'Golo Kante.
"Chelsea have broken up that partnership and strengthened their rivals. All of a sudden United are in the driving seat ahead of Chelsea to win the Premier League.”
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Phil Neville thinks Chelsea will be hurt by the sale of Nemanja Matic
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Neville thinks Matic’s departure could prove costly, adding: “Matic has intelligence and experience of the Premier League. Bakayoko has not and I saw him in the Champions League last year.
“He was good in some games, inexperienced in others. He will take time to settle.
"I think the [Matic] sale at this moment in time, with that Chelsea squad, could cost them the league."
