Manchester United manager, Erik ten Hag has been warned he is “playing with fire” with his new coaching appointments at Old Trafford. Ruud van Nistelrooy and Rene Hake are set to join Man United’s backroom team as part of Ten Hag’s coaching staff. The pair had agreed a two-year deal to become Ten Hag’s assistant at United. However, former Dutch footballer Johan Derksen has now sent a warning to Ten Hag, telling Voetbal: “He (Ten Hag) is playing with fire. “I understand it all. The English staff apparently doesn’t appeal to him, that is why he wants all Dutch people. “Hake doesn’t make you a ch…