The Spanish left-back joined on a temporary basis last August and made a positive contribution during the first half of the season.
Sergio fulfilled the role he was brought in for: to provide cover for Luke Shaw and Tyrell Malacia while both were out with long-term injuries.
While Luke has missed the past two games with a minor injury, the England international is expected back soon. Tyrell, meanwhile, is getting closer to a return from his knee injury.
The versatile Diogo Dalot can also provide further cover at left-back.