The Dutchman, who spoke to Sky Sports News outside United's training ground this morning, also revealed that the club's staff only found out about the decision today.
"Just this morning, he called us into his office and he said what decision he took," said the 49-year-old.
"It has always been on the cards and speculation every season, I think he felt the time is right and he has made that decision.
"He's obviously a man that thinks things through very, very well. I wish him all the well, he has been absolutely fantastic for this club."
It is understood that a successor will be put in place within the next 48 hours.