The Champions League winner says he thought seriously about giving up after being played out of position while with the Dutch giants.
He told RTV Rijnmond: "I really didn't like it. 'Why am I even bothering?', I thought.
"Do I have it? But it's never come this far, I've thought about it, but I also thought: 'What do I do if I really stop?'
"I actually had no plan B."