"I was very happy for him that he won the game against QPR last weekend," Robben told reporters.
"If you are not winning, you get the pressure but the management at United know what sort of coach they have got and I am very confident he will be successful there.
"I have worked with Van Gaal at Bayern as well as with Holland, so I have known him for a very long time. He is one of the few coaches who can make teams play better. He makes players better. He will get United back into the Champions League next season, where they belong."
Manchester United resume their Premier League campaign when they face Leicester City on Sunday.