Complex sentence: After returning to West Ham, he appeared as a substitute in a quarter final round of the same competition against Aston Villa despite having played for a different club in an earlier round of the competition, he was ineligible to play.
Split into simpler sentences: After returning to West Ham, he appeared as a substitute in a quarter final round of the same competition against Aston Villa. As he had played for a different club in an earlier round of the competition, he was ineligible to play.