The Bayern Munich wing wizard was touted for a move to United last summer, but remained at the Allianz Arena.
Robben, who flourished under Van Gaal's tutelage at Bayern as well as for Holland, believes the Dutch tactician made him the player he is today.
But despite his glowing appraisal, Robben maintains that a move to United is unlikely to happen.
"From the first day he made me feel important," Robben told the Daily Mail.
"He gave me an important role and I had a great first year. I am very thankful to him. He is a coach who makes individual players better. That is rare nowadays.
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Arjen Robben has downplayed talk of a move to Manchester United
If things were not going so well, maybe (joining United) would be the first option
Arjen Robben
"I have a lot of confidence he will be a success.
"He needs time to build his team. When he qualifies for the Champions League — and he has a realistic chance of winning the FA Cup — if that happens, he has had a great season.
"The life I have with my family is so good, the team has been successful.
"If things were not going so well, maybe (joining United) would be the first option, but he knows I am happy."
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