Complex sentence: Díaz made his debut in the Spanish top flight on 22 August 2016, starting and scoring the game's only goal in a win at Celta de Vigo--it was also the first-ever goal for the Madrid side in the competition.
Split into simpler sentences: Díaz made his debut in the Spanish top flight on 22 August 2016, starting and scoring the game's only goal in a win at Celta de Vigo. It was also the first-ever goal for the Madrid side in the competition.