"I spoke to Matthijs one week ago about his future, and that's a secret," Koeman said.
"What I told all the young players maybe thinking about making a step [up] is that the most important thing is to keep playing, keep getting minutes to develop your football.
"He, his family, his agent and those around the player will make the best decision for the player.
"His big quality is that he's only 19 and it sometimes looks like he has the experience of a 25-year-old player.
"That was great, that [Ajax] had success in the Champions League, in the highest level competing against big clubs.
"That enhances the development of young players.
"He's a great professional, really clever."