Brain injury typically takes years to recover from. How long is SD to be with out a working Senator?
Haven't you seen the reports about Sen. Muntz, also from SD, who was absent for 4 years in the early 70's? And Biden, who was out for 8 months in the 80's? And what about Strom Thurmond, who didn't know where he was for years?
An elected Senator can serve out his term as long as he is still alive.