He said: "Some honesty has been said in the dressing room. Of course you have to be honest and truthful with each other after that.
"But the main thing is that we are together and that we want to move on and put it right."
The Dutchman added: "Newcastle are a good side and have a striker in Mitrovic who cost #15m, Shelvey cost #12m, Matt Ritchie #11m - and I could go on.
"But I don't want to make excuses. There were things that happened out there that I wasn't happy with.
"We were very stretched and it was easy for them to counter-attack us - and it's very dangerous when they they are able to do that.
"It was easy for them to get in behind us when though we were playing deep.
"It's hard to take, but I must say that Newcastle are a great team with big players and you could see that."