I have no idea if Greg Robinson is a serious candidate for the DC job, but some people seem to take him as one. Many claim that "his resume as a DC is excellent" and point out that "just because he wasn't a good HC, doesn't mean he won't be a good DC". Lets look at those two claims
First, his resume (omitting time as an OC previously):
1 Year with the Jets. Defense ranked 14th in PPG.
6 Years in Denver w/ defenses ranked 17th, 7th, 6th, 6th, 11th, 23rd
3 Years in Kansas City, when their defense was the laughing stock of the NFL - ranked 23rd, 28th, 20th, and he was fired.
1 Year at University of Texas, where he posted decent numbers (28th ranked defense nationally) against a weak schedule (they played 2 teams that finished the season ranked).
While Robinson did win 2 Super Bowls in Denver, his overall resume is what I would refer to as "checkered", and at no point has he coached a defense that could be referred to as "elite".
Second, while the DC job and HC job require different skill-sets, there are some commonalities - like recruiting. He's demonstrated no ability to recruit at Syracuse (which, for the most part, never fared any worse than the rest of the Big East), and has begun to lose 2-star recruits to FCS schools.
Finally, I don't think we should completely throw out his tenure as a head coach, as it was an epic disaster. Syracuse had 13 non-losing seasons in 14 years before his arrival, and had gone to 12 bowls in that time, including the BCS. Upon his arrival, Robinson turned a 6-6 team that returned 17 players into a 1-11 team. In the next 3 years he would reach a high-water mark of 4-8.
This just isn't a "strong" resume. The peak of his career occurred 10 years ago, and it wasn't that staggering of a peak. Since that time, he has been associated with 2 staggering failures (the KC defense, and Syracuse). I know nothing of his X's and O's, and would be unqualified to comment on them, but this guy doesn't seem like "The" guy.