These ratings are given to players based on their performance during the 2014-15 season and range from A+ (the best) to F (the worst). They are given based on how well they performed the role they were given, so they are not absolute but relative to expectation.
Sergio Busquets is no longer football's best-kept secret. In his early days, some dismissed him in comparison to Xavi and Andres Iniesta, his more famous midfield partners. No longer. Busquets now is a known star, and he oozes class every match. Even still, it's fair to say he's a bit underrated even after years of excellence. This season was no exception. He scored an absolutely vital goal with the last kick of the game away to Valencia to win 1-0, which may be his personal highlight. However, looking at that contribution only would be extremely reductionist - that was a singular moment. No, what sets Busi apart was his movement, passing and defense in virtually every game. If there's one thing you can say about his grade, maybe it's that he's set such a high standard for those who know him well that you can't be surprised anymore when he turns in a masterclass.
It's hard to talk about Roberto because it's almost as if no one really knows what to expect. Last season he was unfairly asked to replace Thiago Alcantara after he left for Bayern Munich. He basically failed at that, but he surely wasn't prepared for the task. This year, with reduced expectations, he seemed to pleasantly surprise people. Luis Enrique, a former midfielder, wisely moved Roberto to play mostly in defensive midfield. Roberto, a jack-of-all trades, is no Busquets, but he gives the team energy and tidy passing. Sergi will probably never be a star, but as a squad player, he continues to play his role fine.
Samper, on the other hand, is pipped to be a star and is considered one of the best youngsters at his position in the world. Samper did not play much last season, but Lucho is probably trying to protect him and not rush him too much. He only played 4 games, 3 in the cup, so it's not fair to judge him quite yet. Hopefully, he starts getting more game time soon...
Grade: Incomplete, no grade
Gumbau played only one match, and out of position in defensive midfield, as a replacement for Samper. He did fine, actually, and probably raised his profile. That said, too little to really judge him on.
Grade: Incomplete, no grade.