You don't need to suffer a major concussion to get CTE. Linemen for example still suffer from it and it's believed that the tens of thousands of "micro-concussive" events (ball snaps, rush forward, hit defensive lineman) they accrue in a lifetime of playing football add up to be just as bad or worse than suffering multiple major concussions. Tight ends like Hernandez are often used in blocking situations so he could have suffered something similar.