The Dutchman, brought in following the sacking of David Moyes less than 10 months after taking the job at Old Trafford, is introducing a new 3-4-1-2 formation and increasing the technical demands on his players.
United, who finished outside the European places in seventh last season, play Swansea City, Sunderland and Burnley in their first three Premier League matches but despite a relatively comfortable start, Van Gaal warned against immediate success.
"Every club where I have been, I have struggled for the first three months," Van Gaal told reporters on the club's pre-season tour in the United States.
"After that, they know what I want, how I am as a human being and also as a manager because I am very direct.
"I say things as they are, so you have to adapt to that way of coaching. It's not so easy and also the way I train and coach is in the brains and not the legs.
"A lot of players are playing intuitively and I want them to think and know why they do something. That's a process that is difficult at first.
"When we survive for the first three months, it will be the same as at Bayern (Munich). After three months there, we were seventh (in the Bundesliga) and third in the Champions League (group stage).
"We had to win at Juventus, we did and that was the turning point."
NO MIND GAMES
Van Gaal, who led the Netherlands to third place at the World Cup in Brazil, said he turned down an approach by Tottenham Hotspur before taking the United job, and would not engage in verbal sparing with Chelsea boss Jose Mourinho.
The Portuguese recently hit out at the wage demands of new United defender Luke Shaw, but Van Gaal said he would not let him get under his skin.
"He shall not do that with me," Van Gaal, who appointed Mourinho as his assistant while at Barcelona, said.
"I play against Chelsea and not against Jose Mourinho. My team and his team are playing against each other."
United reportedly spent close to 60 million pounds ($101.59 million) on Shaw and Spanish midfielder Ander Herrera last month but van Gaal said he was determined to give opportunities to the young players already at the club.
"When you see Ajax, I had the youngest team to win the Champions League with 17, 18, 19-year-old players. And the way you see Barcelona still playing with six players from my time is because I gave a lot of chances to the youth players.
"The structure and the culture of the club is Xavi, (Andres) Iniesta, (Carles) Puyol, (Victor) Valdes. Thiago Motta, I also gave his debut and he is playing in Paris St Germain, not a little club.
"At Bayern Munich, it was the same. You can see that in (Thomas) Mueller, (David) Alaba, (Toni) Kroos, that kind of player, so I am always for the long term not the short term.
"When I buy, I buy players for the long term, not the short term because I do respect the club a lot."