 He was also an important unit of the "Célticos" in a 2–0 home success against Atlético Madrid on 15 February of the following year, finishing the season between the posts.In 2016–17, Sergio took part in his first European campaign, playing ten UEFA Europa League matches as Celta reached the semi-finals and lost 2–1 on aggregate to eventual champions Manchester United.The following campaign, under new manager Juan Carlos Unzué, he battled for the starting position against youngster Rubén Blanco.