The forward was left out of Ricky Sbragia's team to face Austria, Croatia and Italy in Vienna, and his father has now claimed that he may have to look beyond a career with the Scottish national side.
"The day after Scotland announced the under-19 squad, and Jack wasn't included, Spain were on the phone asking about his position and international status," John Harper told The Mirror.
"Their national academy is just a few miles from Real Madrid's own training ground, so they know about Jack because they watch his team often enough.
"Jack wants to play for Scotland. I'm Scottish, so is his mum and the rest of the family. He is committed to Scotland, but if nothing is happening by the time he is 21 then he will have a decision to make because he has a life and a career to fulfil. Hopefully, it won't come to that."
Harper was born in Malaga, but both of his parents hail from Scotland.