After fresh optimism at Old Trafford during the close-season, United were brought back to earth on Saturday as Swansea City recorded a surprise 2-1 win.
Van Gaal's side began that game in a new 3-4-1-2 formation but the Dutchman was forced to revert to a more traditional 4-4-1-1 after the break, which saw United improve but ultimately fall short.
For Meulensteen, the manager's indecision on the opening weekend of the Premier League season shows United are still some way off the finished article and he called for the club's fans to be patient as Van Gaal embarks on a rebuilding process.
"This is not going to be a quick fix," Meulensteen - first-team coach under the legendary Alex Ferguson at the club - told talkSPORT.
"They have still got two weeks to go in this transfer window.
"They definitely need to maximise the opportunities in this window and probably do something in January as well. This process will take a long, long time.
"We need to strengthen at the back, we need to strengthen in midfield and that is foremost the two areas they need to look at.
"In my opinion, they need to bring in at least four top players to crank up the level."
United have signed Luke Shaw and Ander Herrera during the close-season but have failed to sign the likes of Arturo Vidal, Mats Hummels and Marcos Rojo, despite repeated links.
