Manager Van Gaal claimed United have “more than enough” defensive cover and quality, despite Chris Smalling’s groin problem further stretching resources.
Robin van Persie’s brace sneaked United past Southampton 2-1 at St Mary’s on Monday, but the Old Trafford side struggled tactically and defensively.
Van Gaal shrugged off criticism of his defensive stocks after switching England midfielder Michael Carrick into a makeshift defensive role midway through Monday’s clash.
“When you have injuries you cannot solve the problem otherwise,” said Van Gaal.
“Do we have to buy players? No. We have more than enough.”
Van Gaal’s terse defence of his squad depth did little to mask United’s continued rearguard shortcomings.
Van Persie converted United’s only two chances on the south coast as Van Gaal’s men moved third in the Premier League with a fifth-straight win.
Southampton pressed continually, pulling Van Gaal’s systems and structures apart almost at will, though too often failing to deliver the killer blow.
Marouane Fellaini’s inept performance forced Van Gaal to haul youngster Paddy McNair off before half-time, in the hope Ander Herrera could add midfield bite.
That led to Carrick’s rearguard shift, with fit-again Jonny Evans replacing Smalling after his groin injury.
United are still missing Luke Shaw, Rafael, Phil Jones and Daley Blind, and that injured defensive quartet formed the foundation of Van Gaal’s justification of his squad depth.
United host Liverpool at Old Trafford on Sunday, with former defender Gary Neville predicting the clash could descend into pub-league standard.
England coach Neville claimed United “got away with murder” at St Mary’s, before tipping the Liverpool clash to resemble “The Dog and Duck versus The Red Lion”.
The unimpressed Van Gaal warned Neville to “pay attention to his words”, asking reporters to interpret his comments however they saw fit.
Neville later denied talk of a rift with Van Gaal when asked about a “feud” on social media.
Van Gaal admitted he hopes Van Persie is now approaching top form after a post-World Cup lull, but waved away his side’s growing momentum as little more than whimsy.
“I hope that he is improving, I have to say I was very pleased with his performance and also his goals,” said Van Gaal of Van Persie.
“The second goal was not so easy, because the ball from Wayne Rooney was coming for his right foot and he took it with his left; so it’s a nice touch. Momentum, things of this sort, these are rhetorical questions. “It’s fantastic to be third in the table, but I had hoped we would manage that with better performance.”