The former England midfielder is concerned that Louis van Gaal's side is "in a desperate situation" and has urged the club to sign more players.
After a disastrous 2013-14 campaign that saw the club finish seventh in the Premier League table, United sacked David Moyes and appointed former Barcelona and Bayern Munich coach Louis van Gaal.
Despite an impressive U.S. tour, United kicked off the new campaign with a 2-1 home defeat to Swansea City and Scholes is concerned that the club could go years without winning trophies if it does not drastically improve.
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"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 column for The Independent.
Executive vice chairman Ed Woodward has been criticized for United's lack of success in the transfer market this summer and Scholes believes that Van Gaal still needs five more players if he is to turn things around at Old Trafford.
"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 criticize the club which I gave my life to as a footballer. But United need to arrest their decline."