I really do not care about ratings, for me If a player scores make assists, and work well in defense, stays on the Team. Of course you need to see the games, to try to see those things.
I had a Mc with ratings of 6 and were scoring a lot, so ratings most of the time do not make sence.

If a player always gets 4-5, get injured 3 or more times during the season, whend i am seeing the game always makes errors or misses chances, sees a lot of cards, i sell.