Sandler is set to join Anderlecht on a season-long loan this week.
Van 't Schip was PEC coach when he was sold to City and told HLN: "There was interest from AZ and PSV after his second season at Zwolle.
“So I tipped him for City. They followed him and also decided to sign him. Unfortunately he was injured when he left for Manchester. He needed surgery on the knee and was out for five months. He then joined the A-team and was allowed to make his debut in the cup competitions.
"If he stays fit, Anderlecht will have a good central defender with a lot of potential. He is also a very good boy in the dressing room and empowered on the field. He has that from Ajax. It is a great advantage that he ends up with his mentor (Vincent) Kompany again."