Scholes also believes Bayern Munich's Toni Kroos should be Van Gaal's first signing as he begins the rebuilding job at the 20-times English champions following a seventh-placed finish in the Premier League this season.
"He's been at the biggest clubs in the world but he has a major job on his hands at Man United," Scholes said in his column on betting website Paddy Power on Thursday.
"It's a massive task and there's a big gap to fill to catch Man City, Chelsea and Liverpool next season.
"United are a long way behind. If Van Gaal gets them to the Champions League, it'll be a start."
The club's executive vice-chairman Edward Woodward was instrumental in selecting Dutch national manager Van Gaal following the sacking of David Moyes near the end of a woeful season for the Old Trafford club.
While supporting the appointment, Scholes said Woodward had much to prove and was facing a crucial summer in the transfer market after failing to strengthen the side last year.
"Edward Woodward has an awful lot to prove this time that he's good enough at his job," Scholes said.
"He has to bring the players in that the new manager wants. It's obvious that last year he didn't manage to do that. If he doesn't, we are not going to get anywhere near the top.
"The first player I'd bring in is Toni Kroos. He's a top-class central midfield player but United need five or six to get anywhere near the top of the Premier League again."
Van Gaal will have Scholes's former team mate Ryan Giggs as his assistant next season but Scholes, who is held in similarly high esteem by the United fans after a glittering career, said he is not likely to be part of the backroom staff.
"I've not spoken to Edward Woodward. I came back for Ryan Giggs for the last few games to try to help out but I'm not waiting for a phone call and don't expect to be at Old Trafford next season," Scholes said.
Scholes was also critical of the decision to sack Moyes less than a season after he was chosen to succeed Alex Ferguson.
"The players did not perform last season," he said. "The manager was a little negative with the teams he put out but I wasn't around the place in training until Ryan asked me back, so I can't judge properly.
"David Moyes took a lot of stick, but I believe he's a top manager. I'd question if 10 months was enough time."