Martial signed from Monaco for a reported £36m on deadline day and van Gaal has explained why Man United paid so much for the teenager.
With Martial just 19 years old, the transfer fee was bound to raise some eyebrows.
However, van Gaal admitted the current market raised the Frenchman’s fee.
“The crazy world of football is that there is a market and a market price. We cannot have any influence on that price. I have said to Ed [Woodward], he is the best of his age, and we need a striker in the future,” van Gaal said.
The Dutchman also felt that had United waited to secure Martial’s services, they would have had to fork out even more.
He continued: “We can wait a year and then he is £10m more expensive, but now we can build him up and at the right time he can enter the game.”
The manager urged fans to be patient with Martial, given his age and his inexperience. Van Gaal is eager to give the forward time to let him “adapt” to the Premier League.
Van Gaal said: “I can imagine that the fans shall think £50m, he has to score! No, he is 19 and he has to adapt. It is very difficult to adapt. You have seen how [Angel] Di Maria struggled and he is 27. [Radamel] Falcao was 29. At Manchester United the pressure is much higher than another club, so I don’t think we have to put too much pressure on him.
“He has to adapt to our culture and our philosophy. Then, maybe he can show signals of his quality in the season.”
Many players have struggled to come to terms with van Gaal’s coaching methods in the past and so the manager will be hoping that while Martial is still young, he can mould him into the sort of player he wants.