There's a ton they can do...upper body and core strength training, flexibility and coordination, aerobics for endurance. There's actually only a limited set of things a person with a lower leg injury can't do.
So they can only do wrist curls? He would be an odd looking D-Lineman.But do they is my question. Seems like someone who has been sidelined for an entire pre season and season with a leg injury, should be looking like Popeye !