A backup needs to be a reliable player, someone who might not have the qualities of the starter but isn't such a downgrade that the opponent can target you and break the weak link. Garcia is a good backup in that regard, although not great with his RB positioning, but as a CB he's a nice 4th option, can even play cdm.
Martin is such a downgrade despite the odd decent cross (this is bare minimum for a pro footballer to occasionally have a good moment), that he makes the entire team weak. Not his fault, he's done his best and I don't blame him, but I really don't want to see him in the squad next year unless he makes some miraculous improvement in pre season.