For the first time since taking the reins at Old Trafford, Van Gaal is facing a seemingly uncertain future with United's top-four hopes hanging by a thread, and Van Hanegem, who featured for Netherlands at the 1974 World Cup, has hit out at his countryman's managerial style.
The 71-year-old former AZ Alkmaar and Utrecht boss criticised Van Gaal for his treatment of assistant Ryan Giggs, who was forced to deny any bad blood between himself and the United manager following the team's late 1-0 win over Newcastle United.
Van Hanegem also slammed Van Gaal over Radamel Falcao after the embattled Colombian striker was banished to the reserves midweek, while British record signing Angel Di Maria has been substituted in consecutive Premier League matches as the Argentina star struggles to make an impact in Manchester.
"What I hate to see is how Van Gaal is treating Ryan Giggs," Van Hanegem told The Mirror.
"Louis has more attention for the other geezer next to him. But Louis forgets that we are talking here about Ryan Giggs.
"Louis has played for Telstar, FC Antwerp and Sparta Rotterdam. Giggs was a giant as a player. He deserves more respect than this.
"Falcao has proven over the past few years that he is one of the best strikers in the world. But at United he is being treated as if he was signed from an old fourth division club.
"Sometimes he's allowed to play a few minutes, sometimes he is allowed to start, other times he sits 90 minutes on the bench, or even worse, in the stands.
"I can see there are more players not happy at United. Di Maria was one of the most important links in the chain at Real Madrid. In the World Cup he was the best player in the Argentina side.
"But at United he hasn't been able to pass a ball over more than 25 yards most times."
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