No it has nothing to do with him being weak or fragile, its that hes listed at 6'2" 220LBS playing LB in the NFL. Granted players weight can change all throughout the years and in season, but that would make him the 2nd smallest LB in the league behind Jamien Sherwood. That alone puts him in a high risk category for injury and it is of no fault of his own. Similar situation with Parsons. I don't consider him to be soft, weak, or fragile, but its a given that he is small for the position he plays and is less likely to make it through a full season than a guy like Myles Garrett. This is simply the trade off for getting smaller, explosive players on the field.
By your definition though from your knee tear example Overshown would be injury prone as he is coming off an ACL tear. Someone with an ACL tear is at a higher risk to tear their ACL again, both in the injured knee and the other knee as well. With a second knee injury in as many years he will for sure be given the injury prone label.