In the SEC there is usually a steep, almost Grand Canyon like learning curve that most freshmen will go through. Smart coaches give those young guys time to develop both mentally and physically. Limiting their role, and responsibilities can be a way for some of the more talented rookies to get on the field early in the SEC. A few guys have the physical skills to play right away, but usually on the defensive line or at a skill position like running back. Very rarely does an inexperienced, undersized player make an impact at the linebacker position.
Dre Greenlaw at Arkansas has been able to jump right past some of that learning curve.