Koeman worked as Van Gaal’s assistant at Barcelona around 16 years ago but the pair fell out due to a dispute.
Van Gaal has been tasked with guiding Manchester United back into the Champions League after being appointed as David Moyes’ successor at Old Trafford last summer.
The players have a fear for Mr Van Gaal. That is not always good
Koeman, who is now in charge at Southampton, believes that Van Gaal causes clashes with his players because he “thinks he knows everything”.
Koeman is quoted as saying by ESPN: “When you bring Van Gaal in the house, you bring in quality, but you also bring in someone who thinks he knows everything, and that will cause clashes.
“There is a big difference between him and and me as managers. I was his assistant at Barcelona after the World Cup in 1998.
“I learned a lot of things from him, but the way he handles his players is totally different from the way I work. I don’t put as much pressure on the shoulder of players.
“When you have Van Gaal in charge, it is a fact that there is fear in his team. The players have a fear for Mr Van Gaal. That is not always good.”
Koeman’s Southampton side are just one point behind United in fifth place in the Premier League table.
United slumped to a 2-1 defeat by Swansea City at the Liberty Stadium on Saturday as they dropped down below Arsenal into fourth place in the table.
The Red Devils’ next game is a home clash with Sunderland in the top flight next weekend.
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