If they cannot run they ball, they will not throw it nearly as well. A good portion of Manning success (of course we mean statisitcally and regular season here) is that the passing game was set up James running, and the respect defenses had to pay to it.
Defensively, they are far from good. If the offense is staying on the field for long stretches, and giving them leads, they will statistically look much better than they are (like in 2005) but a dropoff in offense will expose the defense statistically, to where the stats will reflect the true quality of the defense---not good.
The Colts are in for a major step back this year, possibly all the way to 8-8.