The attacking midfielder joined Blues from Spanish side Valencia on 7 August 2019 on a three-year contract
Villalba is considered one of Spain's most exciting prospects.
A product of home-town club Valencia's youth system, Villalba made his debut for their reserve side aged 16.
He soon after made his senior debut, as a substitute, in La Liga - his only appearance for Los Che at top-flight level to date.
In July 2018, he joined Segunda Division side Numancia on a season loan.
He made 39 appearances, scoring four times, and impressed with his performances, which included making seven assists.
Capped at Under-16 through to Under-19 level at Spain, Villalba's career has been followed by several Premier League clubs.
Blues secured his services on a free transfer, with Villalba's talents considered an ideal fit for Pep Clotet's evolving side.