Complex sentence: McKay began his career as a youth player for Kilmarnock where he spent seven years until he was released due to a hip problem which the Kilmarnock medical staff feared would cause him long-term problems.
Split into simpler sentences: McKay began his career as a youth player for Kilmarnock, where he spent seven years. He was released by Kilmarnock due to a hip problem which their medical staff feared would cause him long-term problems.