By Ewan Murray As pressure continues to mount on Xavi at Barcelona, the La Liga champions face a trip to third division Unionistas de Salamanca in the Copa del Rey on Thursday. Barca visit the Spanish minnows in the last 16 of the competition, although they will need to be wary of Unionistas. The Primera Federacion side knocked Villarreal out in the last round, beating the 2021 Europa League winners on penalties. The Catalan giants will be looking to bounce back from a disheartening 4-1 defeat to rivals Real Madrid in the Supercopa de Espana final last Sunday, their second defeat to Carlo Ance…