Cervical stenosis typically means a narrow central spinal canal but is sometimes used more casually to include cervical foraminal stenosis-- narrowing of the windows through which nerve roots exit. Neither one is necessarily a contraindication for playing football. If a player has deformation of the cord itself with "signal change" inside it seen on MRI or has more than one episode of transient quadriparesis, though, he is probably done. I haven't read enough news about DGF to know whether what he has is more central or foraminal/ radiculopathic, but I think word would have gotten out had he experienced quadriparesis, and people wouldn't be sitting the fence if his MRI were that bad.
The above picture shows congenital (central) stenosis which might have been an incidental finding picked up on an MRI. If it is mild, it might be no big deal. If it is severe, the surgery might be laminectomy (remove the back corner walls of the canal) and fusion. It can be made temporarily worse by disc protruding into the canal, in which case the disc is likely removed, typically with fusion as well.
This picture shows a "side" (sagittal) and "bottom" (axial) view of stenosis affecting the foramen and hence the nerve root. In a young athlete, it would not be arthritis (spondylosis when in the spine) but rather disc, treated as noted above if it doesn't improve enough without surgery.
Patellar tendinitis is not usually a surgical problem. Most guys who need their patellar tendon debrided (bits of unhealthy tendon trimmed out) and repaired return to play, but a good 10% never really do. Out for spring, reasonable chance to be ready for full action in fall camp.