Phil Neville and Danny Murphy are not impressed by the Premier League's most fashionable formation.
The Red Devils started their Premier League opener against Swansea with the 3-5-2 system that van Gaal used so effectively with the Dutch national side at the World Cup in Brazil.
United were vulnerable at the back, however, and they fell to a surprising 2-1 defeat to the Welsh side.
The 3-5-2 formation appears to be gaining popularity in the Premier League however, with Queens Park Rangers and Hull City also setting up with three at the back this weekend.
Neville, who spent 10 years at the Manchester club after coming through their academy, says he does not particularly like the new system.
"It's a system that came out of the World Cup, it seems to be the fashionable system particularly in the Premier League this season," he said on Match of the Day 3.
"It's not a system that I'm a massive fan of really. I think it promotes a lot of slow play."
Neville alluded to the Liverpool side he played against in the 1995-96 season, which featured Jason McAteer and Rob Jones as wing backs and employed a slow, considered style of play.
"It was just slow build-up, there was no penetration from wide areas and you very rarely get into crossing positions," he said.
"I played for a club that was brought up on wide players getting into crossing positions and penetration from the middle of the park."
Former Liverpool midfielder Danny Murphy was also not impressed by the 3-5-2 formation.
"I'm not a big fan, I think it can work if you've got very good players and very bright players, mainly through the spine who can talk to others and put them in position," Murphy said.
"It gives you the chance to play two strikers and not get outnumbered in midfield - that's why a lot of managers do like that system.
"I think it's a worry if you get a team who's brave enough to play two wide men and a striker against the three and go man for man, but ultimately it's about the quality of your players."
United do not appear to currently have the personnel to fit the new system. Van Gaal was forced to play Ashley Young as a left wing-back and young striker Jesse Lingard on the right, and he handed 20-year-old defender Tyler Blackett his debut at centre-half.
Swansea's winner exposed United's weakness at full-back, as Young was caught out of position on a deep cross which went over his head and led to Gylfi Sigurdsson's strike.
The Red Devils have been linked to a host of defensive transfer targets this summer, and Luke Shaw should soon return from injury. Until some new faces are brought in, however, van Gaal should perhaps stick to a more conventional system to remain solid defensively.
Can Manchester United be successful this season with the 3-5-2 system?