The Wales youth midfielder will not be offered scholarship forms by United and instead will join Leeds at the end of the season.
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Walker, who is a youth teammate of JJ Gabriel and Kai Rooney, posted to social media: "After 9 years at the club, this season will be my last.
"I would like to thank all the staff and players who have helped me get to where I am today.
"I have made some great memories and friends, but I am excited for my next chapter and a new challenge."