Van Gaal led the Red Devils for two seasons, winning the FA Cup before being sacked at the end of the 2015/16 campaign – a decision that leaves him bitter to this day.
In a wide-ranging interview with the Guardian, the now-retired former Barcelona and Bayern Munich boss said that Rooney, who left United for Everton in 2017, was one of the best players in his squad despite being past his peak.
“I’m sorry but he was over the hill,” Van Gaal said.
“But in spite of that he was one of my best players.”
Asked about his decision to give Rooney the captaincy, Van Gaal said: “You can say nothing about his professionalism as a player in the training sessions or on a pitch.
“Outside the environment of the training pitch and the stadium is different. I made him my captain because I want to control that life outside.
“I think we don’t succeed totally,” he added.
Looking back on his departure from the Premier League club, Van Gaal explained why he felt let down by his employers, who sacked him a day after winning the FA Cup.
“At that moment I feel betrayed,” he said.
“I feel betrayed because what I hear from a lot of people is it was already done in December, January. I spoke every week with [the executive vice-chairman Ed] Woodward.
“After three lousy defeats in December, when we also went out of the Champions League, I have spoken with him and I said: ‘I can understand that you sack me.’
“In a club like Manchester United, you cannot lose three times in a row. Then he said: ‘No, never. I never shall fire you. Believe in yourself. Don’t read the papers.’
“I think then you can feel betrayed but now I can better understand because he knew that the next year I would say goodbye.
“On the market was [Jose] Mourinho. He is also a very good manager and Woodward thought that he had secured for Manchester United Mourinho’s top managerial level for years.”
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