The midfielder, 16, is moving to City from next season.
"We find it unfortunate that talented players leave us for abroad," Hertog told Feyenoord TV. "But we have done everything in our power to keep him.
"We have been in long conversation with him. And it's not just about presenting a professional contract. But at some point the players must decide."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball