"Whether his team plays good or bad, whether the public and media slaughter him or not, he is not affected by anything," the Daily Mail quotes Truus as saying.
"He always says to me: 'What are you worrying about? Let them have their moment of fun. Only you and I know how things really are. They don't.' The only thing that bothers him is when he sees me getting hurt or upset."
Van Gaal will begin his reign as United manager at home to Swansea City on the opening day of the Premier League season on August 16.