The young defender, 21, signed for City in January and will join the Premier League giants this summer.
"PSV was not very concrete in the winter break, but AZ were," Van 't Schip told Studio Voetbal.
"I then called technical manager Txiki Begiristain of Manchester City and informed him that he is a promising player. The others not so much."
Van 't Schip is close to City, having been head coach of Australian partners Melbourne City last year.