The Cowboys have had a very odd year with regards to their defense. So much so that at times you do just look at the personnel available and concede that to judge these players against a normal grading system is just unfair.
Then, you admit that injuries happen all of the league and there are no sympathy cards arriving in the mailbox. You also see that you are in the middle of a playoff push and you make do with what you have.
Some of the most endearing characteristics of recent Super Bowl Champions is the "next man up" philosophy which basically says that every injury is "just a flesh wound" to the total body of the roster, and therefore, grab a new body and throw him out there.
Of course, we know that this is merely a band-aid and there is a reason that different players get different salaries - because the difference in ability levels between your starter and the free agent off the street that has to replace him in Week 15 is substantial. Or, at least, you hope it is.