After romping home to win the 2012-13 Premier League title, United uncharacteristically finished seventh last season, with manager David Moyes losing his job as a result.
Dutchman Louis van Gaal has since taken charge of first-team affairs but old habits were evident at Old Trafford as United lost their Premier League opener 2-1 to Swansea City.
And Scholes - an 11-time Premier League winner with the club - believes United could continue to decline just like arch-rivals Liverpool did in the 1990s, having claimed the 1989-90 top-flight title before managing to earn just four runner-up medals in 24 seasons since.
Liverpool also finished outside the top three on 15 occasions during that period.
"I am scared for United. Genuinely scared they could go into the wilderness in the same way Liverpool did in the 1990s," Scholes wrote in his exclusive column for The Independent.
If United are to prevent a downward spiral, they must add to their signings of Luke Shaw, Ander Herrera and Marcos Rojo, according to Scholes.
"What do they need? Five players. Five proper players who can hit the ground running and turn round a situation that looks desperate," he continued.
"Let me be clear: I am sick of having to criticise the club which I gave my life to as a footballer. But United need to arrest their decline."
