Lami, that is to prevent your players to get in bad shape/condition. Try playing several games without any condition regulation and you'll see that your players will have quite so sucky condition once you start a match. My players mostly got red up-arrowed and yellow up-right-arrowed condition once I start a match. That is because I perform a condition check on the players I will be using for the upcoming game. And believe me, you don't want to play with a player that got a white down-arrowed condition... he'll be TOO crappy.
Theo, it's not just about scoring and assisting. You have to perform well. Do good runs, passes and not instantly run and try to prevent losing posession. That should help a bit. Like I said above; do regular condition checks as well. Your player also depends on the developement sheet. If your player reaches his developement limit, he WILL start dropping down on his stats. With Adriano, I think that you can go at least 3 seasons before his stats will dramatically go down (haven't tried 3, just 2). The age matters too, of course.
Doing the trainings and matches properly WILL cause a player to boost his stats (well, not 100% of the time, but mostly).
Tip: Fill your team about 40-60% of 15-18 yearies and train them up to be godlikish players.
Good luck.