Hey gkassimis, there are two training elements that always increase a player's core attributes instead of being added to an Ability/Role currently in training. These are the Man of the Match title (for the highest rated player in the winning team), and the daily Training Gift. Both of these add 4% to a player's attributes, and always to his attributes. Never to a Role/Ability.
Since you say that he gained quality after the friendly match you played then he was the top performing player from your team.