Depends on the position and the player.
Average is 6.5-6.9. Anything below that and I tell the player I'm disappointed with them afterwards. If I have a star striker hovering around 6.3-6.5 then I keep them on as they're still able to make a difference but if they continually miss chances and I need a goal you're better off just subbing them. If a defender is having a low 6 game then sub them off immediately if they don't have a good reaction to your HT team talk.
Anything above 7 is good really. For a keeper 6.7-6.9 is good and they'll usually have the lowest ratings on the team. If your tactic uses a deeplying playmaker who doesn't get many goals or assists but does his job then often he'll hover around the 6.9 mark. Might seem average but you have to look beyond the rating and look at his match stats to see if he has a good game because the rating system is a bit flawed.
If one of my attackers has recent form of 6.6-6.8 then I'll bench them but I'm very demanding. Most of my attackers finish the season with 7.7+ ratings and some strikers even 8+ ratings so performing at 6.6 isn't good enough for me. For others it will be good enough, it all depends on the players you have and the situation you're in.