The 21-year-old is set to join the Red Devils next month after he passed a medical with United on Monday.
United agreed a deal to sign Depay from PSV Eindhoven earlier this month as Van Gaal moved to begin his summer transfer business early.
Depay arrives at United with a reputation for being one of Europe’s most exciting young attacking talents.
However, manager Van Gaal says that Depay must be given time to adapt to his new club and warned supporters not to expect too much from him too soon.
Van Gaal told United Review: “Memphis Depay [is] a young player who has come through the PSV Eindhoven academy system.
“I worked with Memphis during last summer’s World Cup with the Netherlands and he has great potential.
“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.”
Manchester United, who were held to a 1-1 draw by Arsenal on Sunday, take on Hull City in their final Premier League game of the season this weekend.
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