I agree that Green is good enough. Give him more opportunity and he'll get better.
Disagree with Zombie that yards don't matter. They do matter--if everytime you ran the ball the play went for a touchdown you'd attract a LOT of attention to your running game. If everytime you run it goes for -10, not so much.
So, production in the running game has SOME impact on attracting the D's attention away from your passing game--the better the production the more distracting.
What production do you need to gain to get the D's attention? I'm not sure, and I'm not even sure how you'd come up with the stats to support a minimum yards per carry.
My expectation is that it would vary based on the situation during the game--if you're winning by 20 the D may be paying more attention to the run because they expect you to run more. If you're down by 20 then it will take a LOT more production from the running game to distract the D from your passing game.
In any event, Green kept the D honest this week, even with the low yards-per-carry.