This is a nice journal article regarding peroneal nerve injury associated with sports-related knee injury.
http://thejns.org/doi/pdf/10.3171/2011.9.FOCUS11187
In this series of cases they looked at nerves that required grafting for repair. Bottom line, the shorter the graft, the better the odds of return of function. The greatest recovery was seen in cases where the graft was <6cm (70% of patients recovered function).
In the case of Jaylon, Dr. Cooper stated that there was no stretching or visible damage to the nerve.
http://www.usatoday.com/story/sport...aylon-smith-knee-nerve-injury-draft/83416394/
My assumption would be that a graft was not necessary due to minimal visible damage.
Hopefully all of this bodes well for Jaylon and his nerve function recovers.