Depends really. If Rafa is out for the rest of the season then Andy needs to win 2200 points between now and the rest of the season to overtake him.
Rafa will be down to 7515 after the US Open. Andy will be down to 6480.
So he needs to win 1040 points from Cinci and the USO to climb above Rafa. I think.
It's all a bit meaningless really though, as it would have bit of a "faux" feel to it due to Rafa being injured, and in terms of seeding would mean almost nothing.
There is of course the possibility that Nadal won't actually get back above him when he returns. In 2009 when Nadal had his problem he didn't really return to his best for a good 6-9 months after he started playing again. This injury is in fairly similar circumstances (unexpected loss in the summer then being out of action for several months with a knee problem).
If it's just a temporary thing where Murray goes above him for a few months then it would be a bit of a hollow rise in the rankings, but it might last a lot longer than his brief stint at number 2.