The new United manager has declared his intention to lead the club to the title after a disappointing campaign under predecessor David Moyes, which saw the side slump to a seventh-placed finish.
Former full-back Neville says Van Gaal is right to be ambitious and feels a lack of European football will help the club improve their domestic form.
However, he says the Dutchman needs to sign world-class players in defence and midfield if they are to offer a realistic challenge to the likes of Manchester City and Chelsea.
“Unless two or three players come in I don’t think it’s realistic that he can win the league this year,” he told Sky Sports.
“Although there is a potential with what Liverpool did in the fact they had free weeks of Saturday to Saturday, continual perfect preparation. That’s a big advantage for United if they get onto some momentum.
“In Chicharito, Rooney, Van Persie and Welbeck, they have still got, I think, probably the best strikers in the league as a pool.
“If they can get the back right and find a solution in midfield – Carrick’s out for two or three months so they need to sign another top midfield player – and stay injury-free then they do have a chance.
“But I do think it would be a massive ask for Manchester United to win the league this year from where they have been, unless they sign two or three real stars and they all bed in immediately in the next few weeks.
“Van Gaal’s got the confidence to think he can win the league. You can’t turn around as Manchester United manager and say anything other than ‘our aspiration and ambition is to win the league’.”
While Neville says United can draw inspiration from Liverpool’s unlikely second-placed finish last season, he does not see Brendan Rodgers’ side repeating that form.
He is tipping Manchester City, Chelsea, Manchester United and Arsenal to finish inside the top four and does not expect the Anfield club to be in the title mix.
“I don’t think Liverpool will win the league,” Neville added.
“I think it was a miracle, really, to get to the point that they did last year. I think they were absolutely at full pelt last year, 100 per cent, to get to the standard that they got to, they really were.
“I thought at one point with three weeks to go that they were going to win it because the momentum was with them, the performances were outstanding and they just fell away at the last.
“Success this year for Liverpool, for me, would look like a Champions League finish, getting to the knockout phase of the Champions League and doing well in a domestic cup.
“That’s not being patronising, I just think that last year was really an over-achievement and I don’t see them being stronger than City and Chelsea.
“Forget United and Arsenal because we don’t quite know what they are going to do yet, but I think the two at the top – City and Chelsea – they’ll find it difficult to get past those two.”
For more from Gary Neville, tune in to the Morning View at 10am Sky Sports News HQ on Sky channel 401 and our live coverage of the Super Cup on Sky Sports 5, Sky channel 406, from 6pm.