Barcelona started their pre-season with a 4-0 victory against third-class Gimnastic on Wednesday night, with substitute Rey Manaj's second half trick stealing the show.Ronald Koeman was without a whole series of his big hitters for the collision at Johan Cruyff Stadium, with the squad members competing in Euro 2020 and the Copa America not yet returned to the club service.FULL TIME! pic.twitter.com/aqLWPPh5fe- FC Barcelona (@FCBarcelona) July 21, 2021Names like Lionel Messi, Sergio Busquets and Pedri were then absent from…