The Dutch manager is keen to play down expectations to allow the youngster adjust to the Premier League.
The young Holland international will complete his move from PSV Eindhoven on 1 July, with a fee having already been agreed and him passing his medical on Monday.
Depay is one of the world’s hottest talents having netted 25 goals in all competitions this season and also starring in the World Cup under Van Gaal last summer.
His arrival is a hotly anticipated one for the Old Trafford faithful, but Van Gaal has called for patience while the youngster adapts to the rigours of the Premier League.
“Memphis Depay [is] a young player who has come through the PSV Eindhoven academy system,” he said.
"The plan is to help him develop as a player, but we must all be patient. The Premier League is very different from any other league and it will take time for him to fully adapt to the pace and unforgiving nature of football in this country.”
The Dutch manager has also said that a good youth policy is important to both him and the club moving forward.
He has relied heavily on some of the younger players within his team this season, with the likes of Paddy McNair, Tyler Blackett and James Wilson all having been given game time, while the likes of Luke Shaw and Adnan Januzaj are also exciting prospects.
And Van Gaal says their first-team experiences will benefit them next season.
“Youth is very important to this club and is a very big part of my philosophy as a manager," he added.
"We have a number of young players in our first-team squad who have benefitted from experiencing first-team football this season and that will help their development going into next season.”