"Without doubt, this is the biggest challenge I have ever faced in football," The Mirror quotes the Dutchman as saying.
"And it is the biggest challenge this club has faced. What we cannot afford to do is lay down and feel sorry for ourselves.
"We cannot do anything about what has gone. We can only get as many points as we can from the 14 matches we have left."
West Ham United and Cardiff City are the other clubs in the relegation zone at the moment.