Van Gaal has hinted he may dip into the transfer market to sign a full-back following the news Ashley Young is set for a long spell on the sidelines.
Young suffered a groin injury in the 1-0 win over Liverpool and will undergo surgery meaning he could miss most of the season, joining Phil Jones, Luke Shaw, Marcos Rojo and Antonio Valencia on the sidelines.
"[Young] must have an operation and you can never tell how quickly the player might return after that," Van Gaal told reporters.
"First of all you have to see if the operation is a success, then you have to make a recovery and see if there is a reaction. It is not something that can be done in a month; it might take longer than that."
READ MORE | Young to undergo operation on 'severe groin injury'
But the Dutchman insists he will not bring in a player as a short-term fix.
"I don't want to buy players for the sake of buying players," he added. "I'm here for the short term – three years – but I will always do everything for the long term.
"I only believe in long-term decisions, what is good for the club and not what is good for Louis van Gaal because he needs a victory on Saturday."
United host Southampton at Old Trafford this weekend with an eye on overhauling fourth-placed Tottenham.