If the tuber itself isn't rotted I think it will be OK. I've actually bought dahlias in bloom from a local nursery in early spring
, cut them back in summer after they bloomed and they regrew and bloomed again fall.
I know what you mean about some doing better than others. I planted 6 tubers of all the same dahlia in 2 pots (3 in each pot). One pot all 3 came up and is full of growth and the other only had one come up and looks pitiful.