Van Gaal endured a nervy start to life at Old Trafford, failing to win any of United's opening three Premier League fixtures.
United were stunned 2-1 by Swansea City in their league opener, while they drew back-to-back games against Sunderland and promoted side Burnley.
The Manchester team were also bundled out of the Capital One Cup, but after spending over £150 million on the likes of Angel Di Maria, Radamel Falcao, Marcos Rojo and Daley Blind, they finally returned to winning ways against QPR last week.
United ran rampant at home to QPR, winning 4-0 and Robben - a transfer target for his former Bayern Munich boss during the recent window - said the club will be playing European football under Van Gaal next season.
"I haven't spoken to him recently, but I was very happy for him that he won the game against QPR," said the Bayern forward.
"Hopefully now he can show that he will do very well, but he needs time.
"I know how it works – 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.
"He will get United back into the Champions League next season, where they belong.
"They are such a big club and that is where they belong."
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