The Catalan club hadn't been at their best in recent weeks, but they returned to form with a 4-0 win over AC Milan in midweek.
Rayo Vallecano have performed well this season, currently occupying eighth place in the league table.
They arrive in Barcelona on the back of a 2-0 success over Espanyol, but they will know that they will need to be at their best to trouble the Spanish giants.
Rayo Vallecano: WLLDW
Possible starting lineups
Barcelona: Valdes; Dani Alves, Pique, Jordi Alba, Puyol; Fabregas, Xavi, Busquets; Messi, Alexis Sanchez, Pedro
Rayo Vallecano: Ruben; Casado, Jordi Figueras, Tito, Galvez; Javi Fuego, Trashorras, Piti, Dominguez; Jose Carlos, Leo Baptistao
Sports Mole says: 4-0