The 15-year-old had spent time in England as United boss Sir Alex Ferguson and Gunners manager Arsene Wenger monitored his ability, while Inter Milan were also credited with an interest.
However, it is Porto who have made the move for the forward and, despite anxiety, he is excited by the prospect of a career in Portugal.
"I believe my playing style is better suited to Portugal," Grey told Sportbladet. "I am a bit nervous but I believe it will go well.
"They (Porto) say they have seen good technique, but nobody with my playing style.
"I am quite big, but I have good technique and a good left foot."