United are suffering a poor start to the season, which sees them just above the relegation places in the league.
One of their significant issues involves a lack of goals, which Neville thinks they can only remedy in the transfer market.
United sold Romelu Lukaku this summer, but did not replace him.
"It won't be a gamble in January as United need to go into the transfer market at the earliest opportunity because they obviously didn't replace Lukaku," the former defender told Sky Sports.
"Those young lads out there - Pereira, McTominay, Wan-Bissaka, Rashford - they have to still buy to a style of play.
"There would be no point signing a replacement for Lukaku that's the same type."