Fabregas is definitely not useless mate. He controls the whole game for me and that is with the likes of Henry, Aguero, Fred, RVP, Fernandez etc etc in the team. Absolutely amazing.
Firstly, NEVER have his "forward runs" setting on often. It should either be mixed or rarely (I have it on rarely). The problem is that in the game you cant do things like irl. Ideally, he should be getting forward because he has the ability to shoot, pass or do anything else you want him to. But in the game the most thing for a player is staying in position. If you keep giving letting him run forward, he'll just keep getting out of position to be useful. So the FR's minimised. Run with ball on mixed, long shots mixed, through balls often and cross ball mixed (if you're playing with a TM) or rarely (if playing without a TM). You can set him as your playmaker as well if you wish (I dont) but that will mean you will have to alter the settings of your other players a bit.
I suggest you go to the tactics forums and have a read through the tactics bible. The CM, AMC and DM positions are the hardest to utilise properly in the game
imo. Its not very easy getting 8's or 9's for players in these positions. Its mostly down to the way the game functions though. Ideally, if a player has a good "decisions" stat (which Fab does), then you *should* be able to set all the bars at "mixed" and leave it to the players judgement. But like I said, in the game you cant do that.
Here's another thing though. If I were you, I wouldnt worry too much about Fabregas's stats. Even if he gets a 6, its not because he's played poorly. Its just down to the AI. He would still have started moves for your team and done all the dirty work. You really realise his true value when he doesnt play.
I'm having the same trouble as you are with Mascherano (different position). He's only gettin 6's and 7's most of the time. But from what I see in the game, he plays very well and doesnt make mistakes. The rating is not always accurate
imo.