New manager Van Gaal decided to use a 3-5-2 formation throughout their pre-season games, using the likes of Antonio Valencia and Ashley Young as wing-backs with Luke Shaw also asked to operate in a slightly different role than he was used to at Southampton.
However United are short of numbers in the central defensive areas with Chris Smalling, Phil Jones and Jonny Evans currently the three recognised senior centre-backs.
Nemanja Vidic and Rio Ferdinand have left the club this summer for Inter Milan and Queens Park Rangers respectively and Irwin feels that the Red Devils are not adequately stocked ahead of the new Premier League season.
United have been linked with Borussia Dortmund's World Cup winner Mats Hummels, as well as Arsenal's Thomas Vermaelen, and Irwin, who played for the club from 1990-2002, thinks that Van Gaal will have to bring some more defensive-minded players to Old Trafford, despite Michael Keane and Tyler Blackett playing on their tour of the United States.
He told the club's official website: "It has been good to see young Michael Keane and Tyler Blackett come into the defence and get some game time too, but I think it's an area we may have to strengthen through the transfer market to give us more experience."
"I think the manager now has a better gauge of the formation and the defenders he has to play in it. I do think some of the defenders need to work on the vocal side of their game. Jonny is vocally strong but a couple of the other lads are a little bit quieter."
"One of them will have to take control and move people around. I'm sure that will come and I'm sure the manager will have encouraged them to be vocal. Any new system needs that but it comes with experience, too. When you're younger, you keep your head down a bit and get on with your game."
"It's up to the experienced players in defence to organise and Jonny's fine doing that."
United have so far spent around £60 million on Ander Herrera and Shaw, but LVG is likely to delve back into the market for a centre-back before the transfer window closes.
The Dutchman's Premier League career begins with a visit from Swansea City on August 16.